The Portland Underground Tunnels. Apparently all over the city of Portland years and years ago were built trap doors, when people fell down them they were locked in cells and held underground to be sold to seafarers for labor. Portland apparently was a central hub for this kind of shanghai, who would've ever thought that?
Cleopatra's underwater palace, Alexandria, Egypt. Explorers are diving to view the ruins of the palace from which Cleopatra ruled, before Egypt was conquered by Rome. Earthquakes and tsunamis have buried the ruins underwater.