What is the deepest lake in the world?

Lake Baikal in Siberia, 1,642 metres deep - which makes it deeper than most of the continental shelf and deep enough to hold about 23 per cent of all the unfrozen fresh surface water on Earth. It contains more water than all five North American Great Lakes combined.

It is deep because it is a rift. Baikal sits on a place where the continent is pulling apart, and the basin is still widening by about two centimetres a year - so it is not a lake in a hollow but a young ocean in its earliest stage, in the same way the Red Sea was a few tens of millions of years ago.

That age and isolation - around 25 million years, the oldest lake in the world - has produced a fauna almost entirely its own. Something like 80 per cent of its animal species are found nowhere else, including the nerpa, the only exclusively freshwater seal, whose ancestors reached a lake 1,700 kilometres from any sea by a route nobody has established. The water is also exceptionally clear, because a tiny endemic crustacean filters it continuously, and in winter the ice is transparent enough to see fish through.