Palladium has a new sourcebook talking all about this race of sub-mariners.
From the announcement:
Rifts® Secrets of the Atlanteans™ is one of those epic sourcebooks with ideas on every page and packed with all kinds of useful and fun information about True Atlanteans, Tattoo Magic, Stone Pyramids, Crystal Magic, Shadow Magic, Atlantean clans, the Sunaj, new Atlantean O.C.C.s, old O.C.C.s revisited, the Shadow Dimension and its monstrous inhabitants, and more. And there are many secrets revealed. Please take a look at the free preview on DriveThruRPG.com for more details and a few choice glimpses of what you can expect. 224 pages, $24.95 retail.