Caspian Sea
The world's largest enclosed body of water, a salty inland sea fed by the Volga
Geographers argue about it, but the Caspian Sea is, by the strictest reading, the largest lake on Earth — a brackish inland sea so vast that the five nations ringing it have spent decades wrangling over whether to treat it as a sea or a lake. Stretching some 1,200 kilometres from the Volga delta to the deserts of Iran, it holds roughly a third of the planet's inland surface water. It is a remnant of the ancient Paratethys, an ocean that once linked the Black Sea and the Mediterranean before tectonics cut it off entirely from the world ocean.
As an endorheic, saline lake the Caspian has no outlet to the sea — the Volga and Ural rivers pour in from the north, and the only way water leaves is by evaporation, which keeps the surface roughly 28 metres below sea level. Salinity runs about a third that of seawater, lower in the north where the Volga dilutes it, higher in the warm southern basin. That southern basin plunges to around 1,025 metres, while the northern shelf is barely a few metres deep. Sturgeon, the source of the world's prized beluga caviar, spawn in its rivers, and the endemic Caspian seal is its only marine mammal.
The Caspian is an oil province as much as a body of water: Baku's derricks helped launch the petroleum age, and the seabed still holds enormous reserves that have shaped the politics of Russia, Kazakhstan, Turkmenistan, Iran, and Azerbaijan. The same isolation that makes it remarkable also makes it fragile — falling river inflow, pollution, and a warming climate are dropping its level and threatening the sturgeon and seal alike. A 2018 convention among the five littoral states tried to settle its murky legal status, splitting the seabed while leaving the surface shared.