Atlantis Endgame

In Earth's future, when time travel has become possible, the Time Patrol is the top secret government agency that protects Earth's past, so that our history will not become corrupted by invaders from either our future or from other worlds. For many years, Murdock, Ashe, and other members of the Time Patrol have contended with threats to our time continuum, none more deadly than the alien Baldies, who hate other high-tech civilizations and want to destroy Earth.
Evidence of time travel has been found in ruins dating to the ancient ... in the legendary realm of Atlantis. So Murdock, Ashe, Eveleen Riordan, and other Time Patrollers deck themselves out as foreign traders to discover whether something is amiss in Atlantis. They find that the Baldies are there, as evidenced by sophisticated, high-tech equipment, whose purpose it is impossible to fathom. As they try to derail the Baldies' plot, the Time Patrollers realize that time is running out on their mission when Atlantis is shaken by tremors that presage a cataclysm that may be the disaster that sank the fabulous island state.
But they must be sure they act to preserve, not destroy, history - and if they're wrong, it'll be too late ... for them and for Earth's future.