Which lake is the world's largest by area (saltwater counted)?

The Caspian Sea, at about 371,000 square kilometres - larger than Germany, and larger than the next several lakes combined. Whether it is a lake or a sea is not a semantic question: it determines which body of international law applies, and therefore how the oil and gas beneath it are divided among five countries.

Under the law of the sea, a sea would give each state a territorial zone and rights over a continental shelf; under lake rules, the whole thing would be shared. Iran wanted lake rules and an equal fifth; the others wanted sectors. A convention signed in 2018 after two decades of negotiation gave it a special legal status that is neither, dividing the seabed by sector while keeping the surface in common - a definition invented specifically so nobody had to lose the argument.

It is also falling. The water level has dropped substantially and models project it dropping several more metres this century, mainly through evaporation and reduced inflow from the Volga - which is dammed along its length. The northern part is shallow enough that a modest fall exposes very large areas, and the Caspian seal and the sturgeon that produced the world's caviar are both in serious trouble.