With prose "as clear and engaging as his ideas" (New York Times Book Review), award-winning author Joe Haldeman blends scientific fact and far-seeing fiction as he pulls readers into a mind-shattering undersea mystery-from outer space...

An unidentified artifact, found seven miles below the surface of the sea, stumps the scientists examining it.

But it calls out to the two immortal creatures who have wandered the Earth for centuries. Two creatures who have never crossed paths-until now...