Lake Tanganyika is a large lake in central Africa (3° 20' to 8° 48' South and from 29° 5' to 31° 15' East). It is estimated to be the second largest freshwater lake in the world by volume, and the second deepest, in both cases after Lake Baikal in Siberia. The lake is divided between four countries – Burundi, Democratic Republic of the Congo (DRC), Tanzania and Zambia, with the DRC (45%) and Tanzania (41%) possessing the majority of the lake. The water flows into the Congo River system and ultimately into the Atlantic Ocean.
The lake holds at least 250 species of cichlid fish and 150 non-cichlid species, most of which live along the shoreline down to a depth of approximately 600 feet (180 m). Lake Tanganyika is thus an important biological resource for the study of speciation in evolution.  The largest biomass of fish, however, is in the pelagic zone (open waters) and is dominated by six species - two species of "Tanganyika sardine" and four species of predatory Lates (related to, but not the same as, the Nile Perch that has devastated Lake Victoria cichlids). Almost all (98%) of the Tanganyikan cichlid species are endemic (exclusively native) to the lake and many, such as fish from the brightly coloured Tropheus genus, are prized within the aquarium trade. This kind of elevated endemism occurs among the numerous invertebrates in the lake, most especially the molluscs (which possess similar forms to that of many marine molluscs), crabs, shrimps, copepods, jellyfishes, leeches, etc.
The lake is situated within the Western Rift of the Great Rift Valley and is confined by the mountainous walls of the valley. It is the largest rift lake in Africa and the second largest lake by surface area on the continent. It is the deepest lake in Africa and holds the greatest volume of fresh water. It extends for 673 km in a general north-south direction and averages 50 km in width. The lake covers 32,900 km², with a shoreline of 1,828km and a mean depth of 570 m and a maximum depth of 1,470 m (4,823 ft) (in the northern basin) it holds an estimated 18,900 km³ (4500 cubic miles). It has an average surface temperature of 25 °C and a pH averaging 8.4. Additionally, beneath the 500 m of water there is circa 4,500 metres of sediment overlaying the rock floor.
The enormous depth and tropical location of the lake prevent 'turnover' of watermasses, which means that much of the lower depths of the lake are so-called 'fossil water' and are anoxic (lacking oxygen). The catchment area of the lake covers 231,000 km², with two main rivers flowing into the lake, numerous smaller rivers and streams (due to the steep mountains that keep drainage areas small), and one major outflow, the Lukuga River, which empties into the Congo River drainage.
The major inflows are the Ruzizi River, entering the north of the lake from Lake Kivu, and the Malagarasi River, which is Tanzania's second largest river, entering in the east side of Lake Tanganyika. The Malagarasi pre-dates Lake Tanganyika and was formerly continuous with the Congo river.