WASHINGTON (AP) — Earth's ultimate survivors can weather extreme heat, cold, radiation and even the vacuum of space. Now the U.S. military hopes these tiny critters called tardigrades can teach us about true toughness.

These animals are pipsqueaks, only about the size of a period. Under a microscope they look like some combination of chubby bear and single-eyed alien. And they are the closest life gets to indestructible.

No water? No worries. Tardigrades survive. Antarctic cold, 300-degree heat (150 degrees Celsius), a lack of oxygen, even punishing radiation doesn't stop these animals. They a...