There are hundreds of thousands, maybe even millions, of lakes on Earth, but only a few dozen can claim to be ‘ancient’ because they have been around for a million years or longer. This is because lakes have short lifespans (on a geological scale)! The moment a lake is created (i.e. when a hole in the ground fills up with water) it begins to die, because inflowing streams and rivers, and even the wind, are continuously depositing sediment in the lake, gradually filling it. This sediment accumulates on the bottom of the lake, which becomes increasingly shallow. Slowly, the lake changes into a swamp, and finally disappears altogether. A lake can only become ‘ancient’ therefore if it deepens faster than it fills with sediment. This is the case with lakes that are situated in ‘Rift Valleys’, for example the oldest lake in the world, Lake Baikal in Siberia (approx.. 30 million years old), and Lake Tanganyika (approx. 12 million years) and Lake Malawi (5-8 million years) in the East African Rift Valley. Rift valleys are situated in tectonically active regions. Through these tectonic processes they become wider and especially deeper over time. Rift valley lakes also accumulate sediment, but they become deeper at a faster rate. Lake Tanganyika, for example, is almost 1.5 km deep at its deepest point, but below the water column is 5-7 km of sediment!