Semper Mars — Ian Douglas

The year is 2040.
The Marines have landed on Mars to guard the unearthed secrets " of an ancient and dangerous alien race: Ourselves.

Scientists have discovered something astonishing in the subterranean ruins of a sprawling Martian city: startling evidence of an alternative history that threatens to split humanity into opposing factions and plunge the Earth into chaos and war. The USMC--a branch of a military considered, until just recently, to be obsolete--has dispatched the Marine Mars Expeditionary Force, a thirty-man weapons platoon, to the Red Planet to protect American civilians and interests with lethal force if necessary.

Because great powers are willing to devastate a world in order to keep an ancient secret buried. Because something that was hidden in the Martian dust for half a million years has just been unearthed...something that calls into question every belief that forms the delicate foundation of civilization... Something inexplicably human.