UnderWorld interpreted in a non-urban setting.
From the introduction:
Beneath the surface of the Atlantic Ocean and the Caribbean Sea, in the area commonly known as the Bermuda Triangle, the ruins of Atlantis lie buried. Protected and obscured by twelve thousand years of sediment and erosion, the ruins lie in a hostile place - dark, cold, and filled with enough pressure to crush an unprotected man like a paper cup.
Yet there is civilization in the deep, people living off the remains of the past and the rubbish of the present, hiding in air-filled pockets resulting from varying combinations of natural formation, elbow grease, and secret Atlantean knowledge. They trade and war with one another, with the Tritons who sacrificed their humanity for fins and gills, all unknown to the surface.