The Black Sea is so deep in places that its lower waters hold almost no oxygen.

The longer version

Oxygen enters water from the surface and from photosynthesis, and it is consumed by decay. Where circulation is weak, deep water never gets refreshed. In this basin, fresh river water floats above saltier water from the Mediterranean, and the layering blocks vertical mixing almost entirely.

Below a certain depth, microbes that do not need oxygen dominate, producing sulfides. Organic matter sinking from above settles into the dark zone and is preserved rather than broken down. Such conditions are called anoxic, and they occur in a few enclosed basins worldwide. Sediments laid down there record past climate in fine layers.

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.