The great drying
Before the flood, there was a desert. Around 5.96 million years ago, tectonic uplift closed the ancient connections between the Atlantic Ocean and the Mediterranean Sea. With its primary source of water cut off, the hot, dry climate caused the sea to evaporate over hundreds of thousands of years in an event called the Messinian Salinity Crisis. The basin floor, in some places 1.5 to 2.5 kilometers below global sea level, became a vast salt flat. Rivers like the Nile and Rhône carved massive canyons into the newly exposed continental shelves, some deeper than the Grand Canyon. Left behind on the basin floor were enormous deposits of evaporites—gypsum and rock salt—up to three kilometers thick in some areas. For roughly 600,000 years, the Mediterranean was a series of small, hyper-saline lakes in a deep, dry chasm.
The Atlantic breaks through
The crisis ended abruptly 5.33 million years ago. Subsidence in the Gibraltar region lowered the land bridge that separated the Atlantic from the arid Mediterranean basin. Water began to spill over. What started as a trickle rapidly escalated into the largest known flood in Earth's history. Seismic data reveals a massive, 200-kilometer-long channel carved by the floodwaters. At its peak, the flood roared into the basin with a discharge of about 100 million cubic meters per second—a flow rate one thousand times greater than the modern Amazon River.
The primary evidence for this event is not a visible waterfall but is buried deep beneath the sea. Seismic reflection profiles show a U-shaped erosional channel and vast, chaotic deposits of sediment and rock debris east of the strait. These deposits, known as megabars, are the geological signature of the flood's power. Models suggest that 90% of the Mediterranean basin was refilled in a period as short as a few months to two years. During the flood's most intense phase, the sea level inside the basin may have risen by more than 10 meters per day. The event was not a single, sheer waterfall and more of a rapidly eroding, high-velocity ramp of water descending over a kilometer into the basin below.