It's your typical spelunking saga: You squirm through tunnels no wider than a watermelon in diameter, then rappel over the edge of a 200-foot cliff into a void so black you can't even see your feet, let alone the bottom-assuming there is a bottom. If you survive that ordeal, you'll still have to negotiate the slippery slope of the "Freakout Traverse" or the dreaded crevasse known as the "Great White Rift." A subterranean tour through Lechuguilla Cave in New Mexico might also include other highlights and lowlights: "Misery Hole," "Death Pit," and the "Parallel Universe."

Some explorers enjoy the thrill of probing an uncharted, underground wilderness. Others find solace in the quiet and darkness of America's deepest cave, which extends more than 1,500 feet below the surface. Then there are those who come searching for life on Mars.