The Aral Sea formed as a remnant of the Paratethys, a vast inland sea that once stretched across much of Eurasia. As tectonic forces lifted the Carpathian Mountains and the Caucasus over millions of years, this ancient body of water fragmented into smaller, isolated basins. The Aral Sea, the Black Sea, and the Caspian Sea are all descendants of that single enormous waterway.
The Paratethys: A Sea That Split Apart
Roughly 30 to 40 million years ago, the Paratethys Sea covered a massive swath of territory from what is now central Europe to central Asia. It was separated from the open ocean by the gradual collision of tectonic plates, the same forces that were pushing up the Alps, the Carpathians, and the Caucasus. As those mountain ranges rose, they acted like walls, cutting off sections of the Paratethys from one another.
During the late Miocene (around 5 to 11 million years ago) and especially during the Pliocene and Quaternary periods, the original sea shrank dramatically. The basins that remained filled with whatever water their local rivers and rainfall could supply. The Aral basin, sitting in the arid lowlands of Central Asia, became one of these isolated remnants. Unlike the Black Sea, which maintained a narrow connection to the Mediterranean, the Aral basin became entirely landlocked.
Why the Basin Holds Water at All
The Aral Sea sits in an endorheic basin, meaning water flows in but has no outlet to the ocean. Two major rivers, the Amu Darya and the Syr Darya, feed the basin. These rivers carry snowmelt and rainfall from distant mountain ranges (the Pamirs and the Tian Shan) hundreds of kilometers across desert terrain before reaching the basin floor. Without these rivers, the depression would be completely dry. Any water that arrives either evaporates or soaks into the ground, which is why the lake has always been salty.
Sediment studies from the basin floor reveal at least 18 distinct evaporation stages stretching back nearly a million years, from about 970,000 years ago to roughly 30,000 years ago. During those dry phases, evaporation outpaced inflow and left behind thick layers of salt minerals. During wetter phases, the lake refilled. This cycle of filling and drying has been the Aral’s fundamental pattern for its entire existence.
Thousands of Years of Rising and Falling
Even over the more recent past, the Aral Sea has never been stable. Reconstructions of water levels going back about 11,700 years show a long history of alternating regressions (shrinking) and transgressions (expanding), driven by shifts in climate, tectonic activity, and eventually human water use. The lake has gone through periods where it was significantly smaller than its modern peak, and periods where it was larger.
Researchers previously estimated that a mid-Holocene highstand (the highest water level reached in the last several thousand years) brought the lake surface to about 72 to 73 meters above sea level. More recent archaeological evidence and terrain modeling have revised that figure downward to about 54 to 55 meters above sea level, suggesting the Aral was never quite as expansive during that era as once thought. Over the last 2,000 years, the lake experienced marked oscillations as climate patterns shifted the volume of water reaching it from the Amu Darya and Syr Darya.
Its Modern Size Before the Shrinkage
By 1960, the Aral Sea covered approximately 68,478 square kilometers, making it the world’s fourth largest inland lake. It held around 1,064 cubic kilometers of water, reached a maximum depth of 69 meters, and had a shoreline stretching more than 4,430 kilometers. It straddled the border between Kazakhstan and Uzbekistan.
That 1960 snapshot represented the lake near the top of its natural range. What followed is well known: massive Soviet-era irrigation projects diverted the Amu Darya and Syr Darya to grow cotton and rice in the surrounding desert. Between 1960 and 2018, the Aral shrank by nearly 88%, dropping from 68,478 square kilometers to just 8,321 square kilometers. What had been a single enormous lake split into several smaller, increasingly salty remnants. But the basin itself, the geological depression carved out over millions of years as the Paratethys broke apart, remains exactly where it has always been, waiting for water that no longer arrives.