The Phanagoria Regression is a theory developed by Soviet researchers which argues that the water level of the Black Sea was lower during antiquity.
This is a well founded theory - a significant number of ancient ruins on the Black Sea are currently underwater, such as Dioscurias, later named Sebastopolis, which lays just off the coast of Sukhumi in Abkhazia. Only remnants of its ruins stick out from the sea (bottom right).
Nevertheless, this theory has one major flaw. There is no indication that the Mediterranean was any lower during antiquity than it is today