Perhaps you recall the 1950s classic horror movie Attack of the Killer Tardigrades. The tardigrades were eight-legged creatures of loathsome appearance with diabolical claws and fierce appetites who were almost impossible to slay. Traveling through the vacuum of outer space without any protection did not kill them. Temperatures as cold as -453 degrees F (-272 degrees C or 1 degree above absolute zero on the Kelvin scale) did not kill them. Temperatures as high as 300 degrees F (150 degrees C) did not kill them. Radiation over 1,000 times the lethal dose to humans did not kill them. Being subjected to pressures of 6,000 atmospheres (roughly equivalent to being at the bottom of Challenger Deep) did not kill them. They spread their reign of terror throughout the earth from the bottom of the sea to the highest mountains.
All right, I must confess; I lied about the movie. There is no such film. However, tardigrades (also known as "water bears" because of their ursine gait) do exist and exhibit the above-described characteristics. But for the fact that the largest ones are only about a half of a millimeter long, they would comprise a deadly threat to humanity. Depending on the particular species, they feed either on plants or small beasties (including each other) and are found all over the earth, although they are especially prevalent among moss and lichens.
They have the ability in a harsh environment, such as caused by a drop in temperatures, to become desiccated, shut down their metabolism, and lose 99% of the water in their body. Such a condition is called a "tun" state, and they can remain in this stasis situation for over thirty years and still recover.
They are not completely immortal, notwithstanding the hyperbole contained within the first paragraph of this Factoid. Exposure to the extreme of extremes of temperature, pressure, and radiation will eventually kill them--for example, they can live only for a few minutes when the temperature is near absolute zero.
If you want to see some water bears in action, find yourself a microscope and a nice moist patch of moss.
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