Water Bears are microscopic animals found in water all over the world (a lot more on that later). They are also known as tardigrades, which means 'slow walker' and the name water bear comes from the bear-like way they walk. Aside from hibernation (a lot more on that later), the similarity pretty much ends there. A large adult water bear may be just 1.5 millimetres long and other species less than 0.1 mm. Also they have 8 legs armed with 4 to 8 teeny weeny claws. Their bodies are segmented, giving them a look not unlike a tiny caterpillar and they are in fact distantly related to insects and other arthropods. Their skin, or rather cuticle, even contains chitin (the stuff that makes exoskeletons hard) so they have to moult as they grow.
Water bears find their way around using lots of sensory hairs all over their body, though some also have two tiny little eyes that are like a reduced version of those found in insects. They have stylets like true bugs and aphids, which are used to pierce plant cells, algae or other small invertebrates. Is there a bone in the world smaller than a water bear? Anyway, after the puncture, the fluids can be sucked up as they flow. Most eat plants or bacteria, but some are predators.
One interesting thing is that they are eutelic, meaning specimens of a single species all have the same number of cells. Even youngsters that hatch from eggs have the same number, the actual cells get bigger as the water bear grows rather than going through cell division.
So where are these things found? Well, they need moisture, so they can be found in nice places like lakes and ponds everywhere. When you're this small your definition of 'water' is a little different from us giants so they can also be found on stone walls and roofs as well as lichens and moss, places where tiny little puddles of water accumulate.
In fact, water bears only need a small amount of water to stay active, so where are these things found? Try everywhere! They have been discovered in hot springs at the top of the Himalayas, under layers of solid ice and in deep sea sediment. They've been found everywhere from the poles to the equator, from 6,000 metres up mountains to 4,000 metres beneath the sea and everything in between.
And that's when they're active! They can also go into a state of cryptobiosis, where their metabolism falls to under 0.01% its normal level and water content to 1%. It's like hibernation to the extent that they almost cease to be a living thing, as they await better times. Or a single droplet of water.
In this state water bears can survive almost anything. Several minutes of temperatures as high as 150 °C or days as low as -200 °C. They can even survive a few minutes at -272°C, about 1 degree above absolute zero or 0 Kelvin, a temperature which isn't actually possible. The reduction in water content means that their cells don't freeze and shatter as the tiny ice cubes expand.
They can survive ten years without water, in fact some do this regularly! Even then, they are still Water Bears, not Bears.
They can take the low pressures found in the vacuum of space as well as the high pressures of the deepest of deep ocean trenches. Actually they can withstand 6,000 atmospheres, almost six times the pressure of the deepest of deep ocean trenches, so they've pretty much defeated planet Earth on that front.
They can even take radiation of 5,000 Gy, where 5 to 10 Gy would kill us mere humans.
To test all this out a bunch of water bears were taken up in a rocket in 2007, and exposed to OUTER SPACE for 10 days! Some of them were protected from high-energy UV radiation, while others were left to suffer the full ravages of solar radiation. Some of the ravaged actually managed to survive, while two thirds of the protected survived and some were even able to mate and create viable young! So if these things ever get terribly out of hand and try to take over the world, there's your answer. Simply get rid of Earth's atmosphere and expose the world to the full power of the Sun! Of course, unless Mars holds some surprises it would mean the end of all life on Earth, but still.