Specification: Brand Naturehike Material 30D Waterproof Nylon Capacity 18L Size 23 x 17 x 42cm Folding Size 12 x 7cm Closure Type Zipper Weight 120g Color Black, Grey, Dark Blue Occasion Camping, Hiking, Travel, Cycling etc Features: - Lightweight and water-resistant material, convenient and portable. - Folding design, easy for storage, multi-functions sports backpack for outdoor use. - Adjustable and padded shoulder straps, soft and comfortable carrying. - Larger capacity and higher holding capacity, sturdy and durable. - Specilized for outdoor sports, travel, business trip, journey etc. Package Included: 1 x 18L Foldable Backpack
1.DURABLE---This CAMEL Backpack is made with high quality tear material, provides extra strength and long-lasting performance with the lightest weight possible.. 2.COMFORTABLE DAYPACK---Breathable mesh shoulder straps with plentiful sponge padding help relieve the stress from your shoulder. Widened and thickened S-type shoulder straps and high elastic breathable back support for best ventilation and easing burden.. 3.DIMENSIONS---34 x 18 x 50.5cm. Lightweight (0.64kg) and Roomy(30Liters).. 4.STYLISH DESIGN---When you carry this CAMEL backpack, you can not only do good outdoor activities, and make you look more stylish and attractive.. 5.WARRANTY---Includes 90 days limited warranty for hiking backpack..
Capacity: 40L 1.Adjustable sternum strap 2.Breathable, molded foam back panel 3.S-shape shoulder straps 4.Front adjustable straps can hang trekking pole or tripod. 5.Bottom suspension system: can hang tents\sleeping mats\sleeping bags 6.Unique back suspension system: Let the backpack and the back out of the cooling ventilation space.You will have a unique comfort all the time. Product Parameters Capacity:40L (2241 cubic inches) Item Weight: 2.75lbs (1.25 kg) Product Dimensions: 49*31*20CM About returns policy and our services Please feel free to leave us a message if you have any questions. We will reply to you within 24 hours and offer you our best services. Washing & Maintenance Instruction 1.Water-resistant coating fabric. Please don't wash with liquid detergent or bleach. 2.Clean with wet cloth if necessary. 3.Air dry. 4.Please do not iron it. Package 1*Backpack
33L Outdoor Sport Backpack Unisex Waterproof Camping Hiking Travel Shoulder Bag
Specification: Material: Polyester 190T Size: 43cm * 29cm * 26cm Color: Black / Blue Capacity: 33L (can be placed in your phone / tablet / laptop / A4 Magazine / Wallets / Umbrella / Clothing and other items) Weight: 0.5KG Feature: 1. The multi-pocket design, convenient classification prevent items 2. The side pockets can be placed on bottles and other items 3. Using sandwich breathable suspension system 4. Suitable for hiking / cross and daily use For the environment: mountain climbing / camping / hiking / vacation / fishing / riding Package Included: 1 Backpack
Huwaijianfeng Hiking Backpack 50L Waterproof Backpack Outdoor Sport Daypack with a Rain Cover for Climbing Mountaineering Fishing Travel Cycling
MULTIFUNCTION 50L HIKING BACKPACK:With a capacity of 50 liters(3,000 cubic inches);9 pockets--large front pocket, bidirectional zippered side pockets, and multiple elastic bands for powerful storage (including space for hats);Great for hiking, travel, camping, for both men & women.. A RAIN COVER INCLUDED:The travel backpack with a rain cover;light, comfortable, and adjustable chest and waist belts; ergonomic padded shoulder straps and back support for the best air ventilation and easing;Suitable for a short backpacking trips (2 to 3 days adventures). DURABLE & COMFORTABLE:Made of lightweight durable materials.Padded adjustable shoulder straps and mesh ventilation promise an enjoyable carry.. 50L WATER-RESISTANT BACKPACK:Rip-stop polyester to prevent the water from sipping into the bag; If the heavy rain comes, then rain cover works.. NO RISK PURCHASING:6 months 100% satisfaction warranty guaranteed & easy-to-reach customer service..
Features: 50L Capacity Foam padded hip belt Holds up to a 15.6-inch laptop High quality YKK zippers Rain Cover Included Various pockets such as elastic side pockets, hipbelt pockets, front pockets etc. Specifications: Item type: Hiking Daypack Gender: Unisex (Men,Women) Material: NYLON Color:Black 50L Capacity:50L(45L+5L) Size: Approx. 30*55*20CM / 11.8* 21.7* 7.9in Weight:Approx. 2.4lb Package List: Outdoor Backpack X 1 50L Rain cover X 1 About returns policy and our services. Please feel free to email us directly if you have any question. We will reply to you within 24 hours and offer the best service. Washing & Maintenance Instruction 1. Water-resistant coating fabric. Please don't wash with liquid detergent or bleach. 2. Clean with wet cloth if necessary. 3. Air dry. 4. Please do not iron it. MONEY BACK GUARANTEE: If you're unsatisfied with your Hiking backpack for any reason, simply send it back within 30 days for a full refund.
Specification: Material Waterproof Nylon Gender Unisex Color Black, Red, Blue, Green, Orange Capacity 40L Size 51 x 36 x 20cm/20 x 14.1 x 7.8inch Use Hiking, Camping, Traveling, Skiing, Rock Climbing, Running, Exercise ect. Features: High strength nylon fabric, waterproof, wear resistant and durable. Ergonomic design, breathable, soft and comfortable carrying. Larger holding space for outdoor equipment storage. The shoulder straps are padded and can be adjustable with loops, grab top handle. Note: There will be 1-3 cm normal minor deviation due to manual measurement. The picture is of actual products. But minor color difference maybe exist due to different light and monitor setting. Package Included: 1 x 40L Outdoor Climbing Backpack
Loowoko Hiking Backpack 50L Travel Daypack Waterproof with Rain Cover for Climbing Camping Mountaineering
8 ADJUSTABLE STRAPS- For tying up your pack or hanging sleeping bag, mat, hammock, tripod, and other gears. mesh side pockets,to keep your stuff separate and organized.. It meet size requirements for most airlines, enough room for 3 or 4 days trips.. Comfortable Daypack: Breathable mesh shoulder straps with plentiful sponge padding help relieve the stress from your shoulder. Widened and thickened S-type shoulder straps and high elastic breathable back support for best ventilation and easing burden. 50L WATERPRROF BACKPACK:A rain cover included.The rip-stop polyester material can prevent the water from sipping into the bag;If the heavy rain comes, then rain cover works.. WARRANTY: Includes 90 days limited warranty for hiking backpack..
Features: * Ergonomic padded shoulder straps and back support for best ventilation and easing burden in order to make carrying the backpack more comfortable. * Chest belt and waist belt are available to adjust. * Crafted from water resistant and tear resistant nylon fabric, very much durable. * Rain cover stored in a bottom pocket which could protect your camping bag does not get wet in a raining day. This pocket can also be used for storage of additional small items (like plastic garbage bag etc.) Organized Compartment: -1.Main Space for most of travel and sports necessities. -2.The zipper on the bottom of the bag for you no need to dig or dump stuff out to reach what is needed. -3.Surface Zipper,Special for your cell phone, MP3 player, snack bar and so on -4.Hipbelt Zipper,Large enough for a cell phone, camera,keys,snacks,or small items you like to have easily accessible. -5.Inside Zipper pocket,Safe to keep your important articles. -6.On the sides, there are 2 very deep mesh pockets with elastic collars for holding your water bottels and umbrellas. These are very roomy and secure. Specifications: Item type: Hiking Day pack Gender: Unisex (Men,Women) Material: Water-resistant, rip-stop Nylon Capacity: 50L(45L+5L) Load-Bearing: 40KG(88lb) Size: Approx. 60 * 30 * 20cm / 23.6 * 11.8 * 7.9in Weight: Approx. 1070g / 2.35lb package Size: 22.28x14.72x 3.15in Package Weight: 2.36lb Package List: √ 1 * Outdoor Backpack √ 1 * Rain cover
Specification: Material Polyester Size 48 x 30 x 17cm Weight Approx. 300g Color Royal, Green, Black, Navy, Orange, Rose Gender Men and Women Features: - Grab top handle. - Wearable and durable. - Made by waterproof scratch-resistant material, suitable for outdoor activities. - Adjustable & removable chest belt and hip belt. - The shoulder straps are padded and can be adjustable with loops. - Compatible Movement: hiking, camping, traveling, skiing, rock climbing, running, exercising school ect. Package Included: 1 x Camping Travel Backpack
Specification: Material: Nylon Capacity: 70L Color: Red, Dark Blue, Light Blue Overall Size: Approx. 68x35x20cm/26.77x13.78x7.87" Features: The shoulder straps are padded and can be adjustable with loops, wearproof and durable. Adjustable shoulder strap and waist strap to fit your need. Larger capacity with main compartment, front compartment, top compartment and bottom compartment. Perfect for cycling, hiking, camping, traveling, skiing, rock climbing, running, exercising etc. Package Included: 1 x Outdoor 70L Waterproof Rucksack
Specification: Material Oxford Size 37x15x11cm /14.57x5.91x4.33inch Weight Approx. 200g Color Black, Khaki Features: - Made of oxford cloth material, waterproof and durable. - Big capacity, enough space to storage somethings what you want.. - Adjustable shoulder strap to carry easily, convenient and practical. Note: - Please allow 1-5cm error due to manual measurement and make sure you do not mind before ordering. - Please understand that colors may exist chromatic aberration as the different placement of pictures. Package Included: 1 x Sport Bag
TOURIT Light Travel Hiking Backpack Packable Foldable Daypack Waterproof Back Packs for Hiking, Large Capacity 35L for Men Women Boys Girls to Picnics, Gym, Outdoor, Hiking
LIGHTWEIGHT and DURABLE: TOURIT backpack for man is made from lightweight 1.1lb highly rip resistant and water resistant fabric, comfortable 3mm pad on the back built for long-lasting performance. MULTI COMPARTMENTS: This carry on waterproof hiking backpack features many compartments to carry your daily essentials. 1 main compartment can be used for a laptop or a bladder; 3 front zippered pockets can carry your gadgets; 2 side mesh pockets for your water bottle or umbrella. WATERPROOF: Light hiking backpack with rain cover keep your Back Pack from water, rain, fog and snow, dual smooth zippers across the day pack ensures not to get stuck on the fabric. FOLDABLE HIKING BACKPACK: Folded into zippered pocket and easily put into handbag, luggage and suitcase for saving space. Unfolded from pouch to backpack in second when reach your destination. MULTIPURPOSE BACKPACK: Breathable, adjustable shoulder straps with padding and chest clip help relieve the stress from your shoulder. Perfect lightweight backpack for hiking, travel, backpacking, gym, vacation, cycling, school, camping and casual activities.
TOURIT Foldable Backpack TOURIT foldable hiking backpack is the perfect companion no matter outdoor sports or leisure activities in or out of the city. Also it is suitable to regard as gift for your family or friend. 【Product Specifications】 ■Dimension: Fold: 11.0" x 2.8" x 7.9" ( L x W x H ), Unfold: 11.8" x 7.5" x 20.1" ( L x W x H ), Capacity: 35L ■Weight: 1.1lb ■Color: Black, Blue, Orange, Green 【Compact, Lightweight and Comfortable】 ■Made of highly ripstop and water resistant polyester, provide strengthen and long-lasting performance with minimal weight. ■Heavy duty double smooth zippers across the backpack are convenient to operation on whichever side you prefer. ■Breathable mesh shoulder straps and back of the daypack with 3mm paded to ensure carrying the backpack more secure and comfortable. ■Adjustable shoulder straps, with a chest Clip to help tighten the backpack minimizes the bouncing when moving.■The extra strength provided by the double-layer bottom piece makes it stand upright and more stylish when you are on the way. ■A true space saver. Fold the backpack into its own front pocket for storage, and unfold it when you reach your destination in seconds. 【Multi Compartments and Keep Things Organized】 ■1 compartment that can be used for a 50oz bladder and one tube hole for the tube which can be fixed on the velcro of the shoulder straps.■Three front pockets are good for holding small accessories and for easy access. ■Two side pockets are good for holding water bottles and umbrellas. 【Enjoy your trips with TOURIT Travel Hiking Daypack】■Lightweight yet durable storage bag makes traveling much easier. Very suitable for Hiking, Cycling, Camping etc. ■When not in use simply fold your pocket into itself containing pouch to transform it into a super compact handheld carry bag.
IPRee™ Outdoor Sports Unisex 20L Folding Waterproof Backpack Bag For Climbing Hiking Traveling
Specification: Brand IPRee Material Polyester Size 24x10x43cm / 9.4x3.9x16.9inch Capacity 20L Color Black, Purple, Camouflage Weight 400g Occasion Camping, Hiking, Travel, Outdoor Sports etc Features: - Lightweight and water-resistant material, convenient and portable. - Folding design, easy for storage, multi-functions sports backpack for outdoor use. - Larger capacity and higher holding capacity, sturdy and durable. - Ideal for traveling, camping, climbing, backpacking, skiing and other activities. Note: - The precise color of the items may vary depending on the specific monitor, the settings and the lighting conditions, so it depicted should only be used as an approximate guide. - Please allow some errors of manual measurement, thank you for your understanding. Package Included: 1 X Backpack
IPRee® 26L Waterproof Ice Cooler Backpack With Bluetooth Speaker Outdoor Hiking Climbing Rucksack
Specification: Style A=Backpack+Ice pack+Ice cube tray B=Backpack+Ice pack+Ice cube tray+Bluetooth speaker Material Nylon(Water repellent) Color Blue, Green Backpack Capacity 26L Backpack size 45 x 34 x 20cm/17.7 x 13.3 x 7.9inch(H x W x T) Iced fresh compartment Size: 12.5 x 30 x 14cm/4.9 x 11.8 x 5.5inch(H x W x T) Temperature: 5-19 Weight 550g Features: Water resistant nylon fabric, high strength, wearproof and durable. Larger storage capacity, special seperated compartment for holding photographic equipment. Soft padded design, damping and shockproof. 2 highlight design on the shoulder strap: One is portable mini speaker, the other is phone pack with earphone compartment. Package Included: 1 x 26L Backpack
GREAT MATIERIAL:The hiking backpack is made of Tear Resistant Nylon fabric, provides strengthen and long-lasting performance.. High CAPACITY: With a capacity of 40 liters,Large enough to carry what you need in the trip,including tent, hammock, sleeping bag, clothes,towel and daily gear for any occasion.. HANDY AND LIGHTWEIGHT:Perfect for extended day hiking or 2-3 days backpacking trips, with a 50 liter capacity and weighing just 1.66 lbs ,this packpack is easy to carry with minimal weight.. PERFECTLY SUIT FOR: hiking, traveling, camping,climbing , and shopping ,suit for both men & women and great gift for everyone.. WARRANTY: Includes 90 days limited warranty for hiking backpack..
Specification: Material: Waterproof Nylon Gender: Unisex Color: Black, Red, Blue, Green, Orange Capacity: 40L, 60L Size: (Approx) 49.5cmx32cmx15cm(40L) 60cmx36cmx19cm(60L) Use: Hiking, Camping, Traveling, Skiing, Rock Climbing, Running, Exercising ect. Features: High strength nylon fabric, waterproof, wear resistant and durable. Ergonomic design, breathable, soft and comfortable carrying. Two capacity options, larger holding space for outdoor equipment storage. The shoulder straps are padded and can be adjustable with loops, grab top handle. Note: There will be 1-3 cm normal minor deviation due to manual measurement. The picture is of actual products. But minor color difference maybe exist due to different light and monitor setting. Package Included: 1 x Waterproof Outdoor Backpack
Venture Pal Lightweight Packable Durable Travel Hiking Backpack Daypack
DURABLE. This Venture Pal Backpack is made with high quality tear and water resistant material, provides extra strength and long-lasting performance with the lightest weight possible. The extra strength provided by the double-layer bottom piece makes it very convenient to carry more load on your journeys. Heavy duty two-way SBS metal zippers across the backpack are convenient to operation on whichever side you prefer. Longevity is further enhanced by bar-tacks at major stress points.. COMFORTABLE. Breathable mesh shoulder straps with plentiful sponge padding help relieve the stress from your shoulder. The length of the shoulder straps is adjustable. The chest strap with a whistle buckle help you lock your backpack in place securely.. MULTI COMPARTMENTS and KEEP THINGS ORGANIZED. This backpack features one main zipped compartment, two zipped front pockets and two side pockets. The main compartment provide enough room (35 liters) no matter if it's a day trip or a week long journey. Two separators in the main compartment are convenient enough to help you further organize things. The two front pockets are good for holding small accessories and for easy access. Two side pockets are good for holding water bottles and umbrellas.. LIGHTWEIGHT(0.7LB) and ROOMY(35Liters). A true space saver. Fold the backpack into its own pocket for storage and unfold it when you reach your destination. To avoid overweight charge, simply unfold from your checked bags and use it as a carry on for your excess baggage.. COMPACT. Folds into zipped inner pocket to fit anywhere, and unfolds from pocket to backpack in seconds. A must have for every trip..
Hikpro 20L - The Most Durable Lightweight Packable Backpack, Water Resistant Travel Hiking Daypack For Men & Women
DURABLE. Made with High Quality Water and Tear Resistant Nylon material. Duarable Abrasion Resistant SBS Metal Zipper Avaliable. Reinforced more than 14 places with Bartack Process. This Bag will Last for Many Years.. LIGHT WEIGHT (7.5 oz) and Roomy (20 Liter). Perfect for day trips, vacation, travel, day hikes, school, camping and shopping. 18*12*5.5 Inch. SMALL TILL YOU OPEN IT. Easy to folds up into small pocket (Sandwich size). You would LOVE that you can throw it in your suitcase, purse or car and have an extra bag without it taking up space. The Secret of Never Paying Overweight Baggage Fee!. COMFORTABLE TO CARRY. 20+ Years Experience makes the Most Comfortable Wide Breathable Mesh Shoulder Straps. Easy to Adjust the length and lock firmly. High quality SBS Two-way zipper goes smoothly.. KEEP THINGS ORGANIZED. Three zippered compartments. Main pocket is Large enough to carry what you need. Outer smaller pocket great for quick access such as guide books and camera. Inner zippered pocket which doubles as the storage pouch is perfect place for valuables such as cash, credit cards and passport. Two large side mesh pockets for water bottles or umbrella..
Never Pay Overweight Baggage Fee Again HIKPRO Let You Have Lighter Baggage and Bigger Carry Capacity. IT'S SMALL TILL YOU OPEN IT Easy to folds into small pocket. You would LOVE you can throw it in your suitcase and have an extra bag without it taking up space LIGHT WEIGHT (6.5 Oz) and ROOMY(20 Liter) Perfect for day trips, vacation, travel, day hikes, school, camping and shopping DURABLE Made with High Quality Water and Tear Resistant Nylon material. Most Duarable Abrasion Resistant SBS Metal Zipper Avaliable. Reinforced more than 14 places with Bartack Process. COMFORTABLE AND EASY TO CARRY 20+ Years Experience makes the Most Comfortable Shoulder Straps. Easy to Adjust the length and lock firmly. The SBS Two-way zipper go Smoothly KEEP THINGS ORGANIZED Three zippered compartments. Main pocket is Large enough to carry what you need. Outer smaller pocket great for quick access. Inner pocket which doubles as the storage pouch is perfect place for valuables. Two large side mesh pockets for water bottles. Our Guarantee To You If within 30 days you're not 100% happy with your purchase of HIKPRO packable backpack, let us know and we'll refund your entire purchase price...no questions asked! Important... Read This We Apologize If We're Sold Out. Because Of The Extremely High Demand. Order it Now before it's too late! Click the Yellow "Add to Cart" Button at the Top of This Page to Enjoy Your Happier Trip Now!
Sunhiker Cycling Hiking Backpack Water Resistant Travel Backpack Lightweight SMALL Daypack M0714
Material:Highly tear resistant nylon,water-resistant. MatReflective stripe,volume control system,adjustable shoulder straps. Breathable mesh shoulder straps, bottle holder on the both sides. Suitable for all hiking,camping,climbing,traveling cycling outdoor activities. Dimensions:15.7x9.1x5.9 inches. Weight:12.3oz.
Product Specification: 1.Material: Highly tear resistant nylon,water-resistant. 2.Grade:Superior. 3.Color: blue,black,green,grey,purple. 4. Dimensions:15.7x9.1x5.9 inches. 5.Weight: 12.3oz. Product Feature: 1.Breathable mesh shoulder straps, bottle holder on the both sides. 2.Water-repellent and tear-resistant Nylon Material. 3.It will be a fantastic necessity for not matter outdoor sports or leisure activities in or out of the city. 4.Convenient for hiking,traveling,camping,running,cycling,mountaineering. 5.Suitable for women girls boys teenager students. Package includes: 1 * backpack. Washing instruction: 1.Don't wash with liquid detergent or bleach. 2.Clean with wet cloth if necessary. 3.Dry naturally.
Mubasel Gear Insulated Hydration Backpack Pack with 2L BPA FREE Bladder - Keeps Liquid Cool up to 4 Hours – For Running, Hiking, Cycling, Camping
1.PERFECT SET - Included 70-Ounce (2 Liter) BPA FREE FDA APPROVED food grade hydration bladder, test report will be sent to your e-mail. 2.THERMAL INSULATION - With the insulated bladder compartment, your drink can be kept cool for up to 5 hours, that's cool!. 3.GOOD VOLUME - Includes 3 storage compartments that will fit your clothes, keys, purses, phones etc. the front stretch mesh pockets allows you to pack even more on your hike. 4.100% LEAKAGE PROOF DESIGN -The opening is large design, you will never be bothered to dry the bladder as the screw one, just bottom up, it will dry itself, with click in connection instead of traditional screw one, it will never get loose and causing leakage. PERFECT OUTDOOR GEAR: Padded shoulder straps, breathable system and lightweight design are the perfect tool for outdoor sports, Mubasel Gear offer 60 days 100% Satisfaction Guarantee of the pack and free replacement of the bladder..
Need To Have Enough Water On Your Outdoor Sports? Are You Wish Your Drink Could be Still Cool After Lots of Activities? When going out for outdoor sports, you need to make sure you have enough water for supplement and a Hydration pack that does not leak, meanwhile has enough volume to take your belonging safely along with you, and the thrill thing is the pack that can insulated your drink to keep cool. Now you can have all these come true with owning Mubasel Gear Insulated Hydration pack The Mubasel Gear Hydration Pack Gives you 70 Oz. Guaranteed Enough Hydration for your whole day out Adjustable shoulder/chest straps. fit chest sizes from teenagers/kids with 27" up to adults (men & women) with 50" chest size. The chest straps prevents bounce from exercising Extendable bite valve makes it easy to reach Easily cleaned by using warm water to rinse out the bladder 3 Storage Pockets to safely carry your belongings along with you Mubasel Gear 60 Days 100% Satisfaction Guarantee: If for any reason you are not 100% satisfied with this pack, please contact us within 60 days, and we will immediately refund every penny you paid for - no questions asked. Don't wait for this sale to the end! Go to the top of this page and click the ADD TO CART button before closing of the promotion
Mountaintop 40 Liter Hiking Backpack with Rain Cover for Outdoor Camping
6 ADJUSTABLE STRAPS- For tying up your pack or hanging sleeping bag, mat, hammock, tripod, and other gears.. Computer Compartment:Your Laptop(big enough for a 17"laptop).There is a compartment that can be used for a bladder and a holding place for the tube but no specific bladder space.. It meet size requirements for most airlines.. The hip belt is adjustable but can not be removed.(the size:25--53 inches). -WARRANTY- Includes 90 days limited warranty for hiking backpack.
Features Capacity: 1.Front pocket with an internal mesh pouch Bearing System: 2.Adjustable sternum strap 3.Breathable, molded foam back panel 4.S-shape shoulder straps Attachment Point: 5.Both sides of the shoulder straps has a D-shape hook, in which small items can be linked to 6.Front adjustable straps can hang trekking pole or tripod. 7.Bottom Zipped Access:The bottom zippered access which can easily access to the main compartment. Product Parameters Capacity:40L (2,440 cubic inches) Item Weight: 2.1lbs (0.93 kg) Product Dimensions: 21.7 x 13.8 x 9.8 inches (55 x 35 x 25 cm) Front Zipped Pocket:7.4x6.6x1.5 inches(19x17x4cm) Middle Zipped Pocket:6.6x8.2x1.1 inches(17x21x3cm) Internal Pocket(The Pocket Behind Middle Pocket):8.6x5.5 inches(22x14cm) Side Mesh Pocket:5.9x5.9 inches(15x15cm) About returns policy and our services Please feel free to leave us a message if you have any questions. We will reply to you within 24 hours and offer you our best services. Washing & Maintenance Instruction 1.Water-resistant coating fabric. Please don't wash with liquid detergent or bleach. 2.Clean with wet cloth if necessary. 3.Air dry. 4.Please do not iron it. Package 1*Backpack