Tardigrades, also called water bears or moss piglets, are microscopic eight-legged creatures barely half a millimeter long that can survive conditions lethal to almost all other life forms. These resilient animals have endured exposure to outer space itself, along with extreme temperatures, crushing pressures, radiation, complete dehydration, and starvation that would kill virtually any other organism.

First described by German zoologist Johann August Ephraim Goeze in 1773, tardigrades were named by Italian biologist Lazzaro Spallanzani in 1776, their scientific name meaning "slow walkers" in Latin. Today about 1,500 known species inhabit Earth's most extreme environments: mountaintops, the deep sea, tropical rainforests, and Antarctica. Their ancestors first appeared roughly 500 million years ago in the Cambrian period, making them among Earth's oldest animal lineages.

These plump creatures live in mosses and lichens worldwide and are easy to observe under a low-power microscope, accessible to students and amateur scientists everywhere. Their four pairs of stubby legs end in claws or sticky pads that enable their characteristic clumsy crawling. Unlike arthropods, tardigrades lack several important genes and possess an unusual body plan where most of their structure corresponds to an arthropod's head. Their legendary toughness and distinctive appearance have made them popular in science fiction and consumer culture, appearing on clothing, toys, and in countless crochet patterns.