Lake Tanganyika is the world’s longest (670km) and second-deepest (over 1400m) fresh-water lake. At somewhere between nine and 12 million years old, it is also one of the oldest lakes on the planet and, thanks to its age and ecological isolation, is home to an exceptional variety of fish. Most notable are its colourful cichlids, many of which are found nowhere else, and which make for some wonderful snorkelling in the lake’s clear waters.