From Dynamites Warlord of Mars.

The Omean Sea is a large body of water hidden about two miles beneath the surface of Mars. The seas receives its waters from the smaller sea above it Korus. One entrance to this subterranean world is a thousand foot in diameter hole upon a black mountain, located near the Valley of Otz. The subterranean world is governed and maintained solely by the first born, who built four great pumping stations to keep it from filling to a certain level, this water is redirected far to the north were it is emptied into the irrigation system built by the Red Martians! The sea dotted with many islands, the vegetation of this underground place is colorless. The sea is illuminated by a strata of phosphorescent stone that ensures eternal daylight in this subterranean world.

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