Tardigrades, also known as water bears, can live 10 days in vacuum, which makes them the hardiest animal in existence. They can also survive extreme conditions that would be rapidly fatal to nearly all other known life forms. They can withstand temperature ranges from 1 K (−458 °F; −272 °C) (close to absolute zero) to about 420 K (300 °F; 150 °C), pressures about six times greater than those found in the deepest ocean trenches, ionizing radiation at doses hundreds of times higher than the lethal dose for a human, and the vacuum of outer space. They can go without food or water for more than 30 years, drying out to the point where they are 3% or less water, only to re-hydrate, forage, and reproduce.
The biggest adults may reach a body length of 1.5 mm (0.059 in), the smallest below 0.1 mm.