The water bear, or tardigrade, is the world toughest multicellular animal. Measuring less than a millimeter on average, these tiny creatures survive the most hostile conditions. These fascinating creatures can be found about everywhere -- on the bottom of the ocean, under meters of ice, in hot springs, and on the top of the himalaya! Tardigrades really are miniature animals, having tiny legs, claws, eyes, mouth, stomach, and even nerves. They have such precise muscle control that they can even move like higher order animals. They are able to survive the most extreme environments that would kill almost any other animal. They can take temperatures close to absolute zero and hotter than boiling water, withstand over 1000 times more radiation than humans, can live over a decade without water, endure six times the water pressure in the deepest ocean trench, and even survived in the vacuum of space, making them the only animals to do so.
The key of their remarkable durability is that they are capable of decreasing their metabolism with a factor of 10,000 and decrease their water content to 1% of normal. When conditions get though, they basically die, stopping any process in their miniature bodies for up to 120 years! When conditions get better again, they revive and go on with their lives.
Tardigrades got their name from their miniature resemblance with a bear, as is so nicely put by Johann August Ephraim Goeze (1773) who was the first to ever describe them: "Strange is this little animal, because of its exceptional and strange morphology and because it closely resembles a bear en miniature. That is the reason why I decided to call it little water bear."
Via Dr. Stefan Gruenwald