It does appear that, at some point in the recent geologic past, the Mediterranean Sea may have been at least partially dry. A study conducted in the 1960s found a layer of minerals in the seafloor that could only have been created by the evaporation of water.
In 1961, seismic surveys were carried out on the Mediterranean basin that found a geological feature 330 to 660 feet (100 to 200 meters) beneath the Mediterranean seafloor, dubbed the M reflector. In 1968, the Deep Sea Drilling Project (DSDP) began, supported by Texas A&M University, and while bringing up rock cores from the sea floor, the research vessel Glomar Challenger discovered a layer of evaporites up to 1.86 miles (3 kilometers) thick. These minerals, including anhydrite, gypsum, rock salt, and arroyo gravel, are evidence that the body of water had partially or entirely evaporated in the recent geologic past, during Miocene times, about 5.9 million years ago. This event was thereafter called the "Messinian Salinity Crisis," named after Messinian evaporite discovered on the island of Sicily.
Scientists later pieced together the evidence and determined what happened in the Mediterranean Sea at that time. The Strait of Gibraltar has closed on a cyclic basis at least several times over a 700,000 year period. The layer of evaporites was far too thick to be deposited in a single event, which suggests that the water in the Mediterranean evaporated repeatedly. Even today, the water is evaporating faster than it is being replenished, due to a lack of large glaciated mountains as a water source and its relative disconnection to the world sea. If these waterways were shut off, the sea would evaporate dry in only a thousand years.
Although some parts of the Mediterranean are as much as 3 miles (4.8 kilometers) deep, comparable to the deep ocean, the Strait of Gibraltar's depth is about 1,000 feet (300 meters), still very deep, but possibly changeable. Around 5.9 million years ago, the Eurasian and African tectonic plates would have been closer together, and the strait was likely shallower. As water froze into glaciers, it may have been taken water from the world's oceans and lowered the sea level enough to close off the strait. Deeper changes to the underlying crust from tectonic forces may have been at play, such as by changing overall rock density.
The dry Mediterranean basin would have been a lifeless and hot place due to the high salinity and areas of the geography as much as 3 miles (4.8 kilometers) below sea level. By comparison, the lowest point on land today, the shore of the Dead Sea, is just 1,371 feet (418 meters) below sea level. At the level of the Mediterranean, there would be 1.7 times the atmospheric pressure at sea level. This means a wind blowing there would be 57°F to 85°F (32°C to 47°C) hotter there than at sea level, which may have been scorching. The evaporites covering the entire basin would preclude the presence of any plant or animal life, so the area would have been one of the harshest deserts on Earth.
