Between the 1960s and 1990s, the Amu Darya and Syr Darya rivers that flowed from the mountains to supply the lake were redirected to irrigate 7 million hectares (1.7 million acres) of cotton fields for the Soviet Union.

There you go.

But also

Aralkum’s dust is particularly toxic compared to the rest of the region’s desert dust because it contains runoff from nearby USSR chemical weapons testing and is full of fertilizers and pesticides from the same mass agricultural practices responsible for draining the Aral Sea.