Tardigrades — also known as water bears or moss piglets — are pretty adorable. Under a microscope, each one looks a bit like a plump caterpillar, wriggling around on eight legs tipped with dainty clawed feet. But don’t let their cuteness fool you. These salt grain–sized animals are some of the toughest beasts on Earth.

Water bears can brave days in space without air. They can cope with temperatures colder than any ever measured on Earth. They have been known to survive hundreds of times the X-ray radiation that would kill a human. And they’ve even withstood getting blasted out of a gun at more than 1,800 miles per hour. Basically, tardigrades are the ultimate extremophiles.

Why did these wee creatures evolve to be so durable? Some 1,300 species of tardigrades have been found. They thrive in ponds, streams and damp soil and clumps of moss. Many of those places can dry out in the summer. So tardigrades need ways to survive being parched. Adaptations they evolved to survive dehydration may have bestowed tardigrades with the power to survive other harsh conditions.