The Aral Sea
At slightly more than 26,000 square miles, the Aral Sea was the fourth-largest lake in the world, behind the Caspian Sea, Lake Superior, and Lake Victoria. It was situated between Kazakhstan and Uzbekistan. Naturally, all of that is in the past tense as the Aral Sea no longer exists.
The lake contained nearly a thousand islands collectively known as the Sea of Islands. When water levels began to fall in the 1960s, problems started to arise. The cause? Russian Federation. Many irrigation projects carried out by the Soviets involved rerouting rivers away from the lake and toward other areas. The lake had nearly completely dried up by 2014.
The destruction of the lake has radically altered the terrain and is regarded as one of the worst environmental catastrophes ever. The lake, which has since split into multiple smaller lakes, is currently being preserved. The South Aral in Uzbekistan largely died off, while the North Aral became two different enterprises. In some areas of Kazakhstan's North Aral, which is currently being conserved, fish are once more living there.