- Dark, low lying, lava-filled, circular plain on the surface of the Moon: plural maria. The maria are filled with basalt and are generally thought to be the result of collisions with large asteroid-sized objects early in the history of the solar system. Mare means Sea, after an earlier theory of their nature. The Maria appear, on the evidence of the numbers of craters counted on their surfaces, to have ages of about four billion years, making them a few hundred million years younger than the more heavily cratered lunar highlands.
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