Lake Baikal is the deepest lake on Earth and holds very ancient water.

The longer version

A rift valley forms where the crust stretches and a block drops down, creating a long, narrow basin. Baikal sits in such a trough, and its floor lies well below sea level even though the surface is above it. Sediment has accumulated over a very long time, recording past climate in layers.

Depth and volume combine to give slow water turnover, meaning the whole body mixes only gradually. Cold, oxygen rich water reaches the bottom, supporting unusual species found nowhere else. Many of its plants and animals are endemic, adapted over long isolation to conditions in this single lake.

Where this entry sits

These facts wander across maps, borders, deserts, rivers, mountains, and the extreme corners of the natural world. Each one is short and self-contained, offering a small surprise about places you may never visit rather than anything you actually need. The full list sits on the Geography page, where every entry is listed in order.