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Temporal Navigator
Mars Odyssey spacecraft relayed images of the dark, nearly perfectly circular features ranging in diameter from 328 to 820 feet.
The holes, which the researchers have dubbed the "Seven Sisters," are at very high altitudes on the planet, on a volcano named Arsia Mons near Mars' tallest mountain. They probably formed as underground stresses around the volcano caused spreading and faults that opened spaces beneath the surface.
The find has led some to wonder if these or other caves on the planet may provide shelter to life or former life on the Red Planet.
Discovery Channel Source