They're tiny animals that grow up to one millimeter long, and have eight legs.

They're also known as water bears.

Their feet look like this, more like claws.

And up close, their faces look like this.

Some speculate that there are around 900 known species.

They were given the name Tardigrada, by Italian biologist Lazarro Spallanzani.

They're also known as moss piglets.

They typically live around water and damp moss or lichens, but can be found almost everywhere on Earth.

Some theorize they might have come from space, of course, this is just a theory.

They can survive dry periods by curling into little balls.

They can hang around like that for 100 years, before casually regenerating.

Some species are carnivorous.

Some might even eat other Tardigrades.

So, how resilient are these little guys?

They can withstand 1,000 times the lethal human dose of X-ray radiation.

In 2007, some Tardigrades hitched a ride on a space shuttle and lived outside the spacecraft in an oxygen-deprived vacuum of space for 10 days.

Many survived.

All of which means, it's probably impossible to kill a tardigrade.