What is the world's saltiest sea open to the ocean?

The Red Sea, at around 40 parts per thousand against the ocean's 35 - which is saltier than it has any obvious right to be, given that it connects to the Indian Ocean. The reason is that almost no fresh water arrives: there is not a single permanent river flowing into it, it sits in one of the hottest and driest regions on Earth, and evaporation is enormous.

It is also getting wider. The Red Sea is a rift where Africa and Arabia are pulling apart, and it is an ocean in its infancy - in tens of millions of years it will be a full ocean basin. At the bottom are brine pools, dense pockets of water so salty they do not mix with the sea above and have their own visible shoreline underwater, in which almost nothing lives and which preserve anything that falls in.

That combination - warm, salty, stable - makes it one of the most heat-tolerant reef systems anywhere, and its corals are being studied intensively because they survive temperatures that bleach reefs elsewhere. It also holds Bab el-Mandeb at its southern end, a strait about 30 kilometres wide through which a substantial share of world trade passes, which is why disruption there in 2023 pushed shipping around the Cape and added ten days to Asia-Europe voyages.