A lost human colony on a volcanic world whose people were shrunk to a fraction of an inch by a natural radiation, discovered in the Animated Series episode The Terratin Incident.
Terra 10 is the forgotten colony behind the Animated Series episode The Terratin Incident. The Enterprise picks up a faint signal in an obsolete code containing the single word Terratin, which turns out to be a worn-down form of the colony's name, Terra Ten.
The settlers' descendants live in a miniature city on a planet racked by volcanic eruptions. Spiroid-epsilon waves given off by the local crystals have reduced them over generations to less than a sixteenth of an inch in height. Unable to call for help in any other way, they bathe the Enterprise in the same radiation, and its crew begins to shrink.
Kirk finds that the transporter restores a person to full size. With his crew cured, he beams up the whole city and moves the Terratins to the safer planet Verdanis.