The typical black bear has a sense of smell seven times stronger than a bloodhound's, and an appetite to match. Bears are omnivores and opportunists, always hunting for more food to pack down before their long winter nap. On top of that, all bears seem to have a killer addiction to junk food.
Some parks, especially in California and Alaska, require backpackers to store food in sealed bear-proof canisters. But even in places where canisters aren't specifically required, it's still a good idea to practice caution. Bears, food, and humans don't mix. Once a bear has grown used to humans, it's especially prone to midnight campground raids and aggressive behavior. Black bears have been known to drag sleeping humans out of their tents, simply because the camper's pajamas smelled like leftovers from dinner.
Ursack makes its bear-proof bags from ultra-strong fabrics like Spectra and Vectran. Weighing in at 8 ounces, the bags provide a food-storage capacity of 650 cubic inches (10.5 liters).
Sometimes Ursack has difficulty keeping its Spectra S29 model in stock, since the material is in high demand by the U.S. Military. The V27 is a great alternative if they're out of the S29. It's almost as strong, and weighs half an ounce less.
The Ursack ships with a special odor-proof ziplock bag. Unfortunately these plastic bags have a tendency to quickly wear out. Obviously, holes make an odor-proof bag somewhat less than effective. Replacements can be purchased in three-packs for about $11.
Even with a defunct odor-barrier, my Ursack has not yet weathered a bear attack. It has, however, held its own against squirrels, mice, and various other rodents. In the end, if all it's done is prevent a scheming squirrel from chewing a hole in my pack, I think it's worth it.
If you'd like to see an Ursack take on a 1500 pound grizzly, check out the video below.