Shadow zone / Iris Johansen and Roy Johansen.

Enhanced descriptions from Syndetics:

The #1 New York Times bestselling author teams up once again with her Edgar award-winning son to bring us a new heart-pounding thriller involving the search for a mysterious Atlantean city While mapping the ancient underwater city of Marinth in the Atlantic Ocean, submersible designer Hannah Bryson makes a shocking discovery: she may have finally uncovered the truth about the once-glorious city's mysterious demise. Long ago it was a thriving metropolis bursting with culture and life--Marinth's sudden disappearance has plagued scientists for years. But now, with her unexpected new findings, Hannah realizes that the ancient city's explosive secret could have dire consequences for the modern-day world. But Hannah isn't the only one who realizes it. When her key artifact is hijacked en route to a research lab, she is thrust into an adventure in which she must match wits against a terrifying enemy who will kill anyone who stands in his way. And when Hannah becomes his target, she knows that her best hope for survival lies with Kirov, a mysterious and deadly man from her past. Together they will race to unravel Marinth's last great secret in order to prevent a catastrophe of global proportions. Before the lethal game is over, however, Hannah will realize that no one is above suspicion.