Cave Entrances Found on Mars Sept. 21, 2007 — Get out your Martian spelunking gear: NASA has found what appear to be seven cave entrances on Mars.
The Mars Odyssey spacecraft relayed images of the dark, nearly perfectly circular features ranging in diameter from 328 to 820 feet.
BIG rabbits on Mars!
At first the dark spheres puzzled scientists, but Odyssey's infrared camera then checked the daytime and nighttime temperatures of the circles and suggested they were, in fact, openings to subsurface spaces.