Lake Tanganyika is so long that if you laid it on a map of Europe, it would stretch almost from London to Paris. It is shaped like a finger, sitting in a giant crack in the Earth's crust called the Great Rift Valley. The lake is around 10 million years old - much older than most lakes, which are usually just a few thousand years old.
Because it is so old and so deep, the lake has its own private collection of fish. Over 250 different kinds of fish live there, and around 200 of them live nowhere else on Earth. The most famous are the cichlids - small, brightly coloured fish that come in every shade of blue, yellow and orange. Many home aquariums around the world keep Tanganyikan cichlids.
Fishing boats have set out on Lake Tanganyika for thousands of years. Today, fishermen go out at night in small wooden boats with bright lamps. The lamps attract a tiny silver fish called dagaa to the surface, and the fishermen scoop them up in nets. From the shore, the lake at night looks like a city of floating stars.
The lake is so deep that the bottom is in complete darkness, with no oxygen. Almost nothing lives down there. Most of the life is in the top 200 metres, where the sun still reaches. So even though the lake is 1,470 metres deep, most of its busy world is near the surface.