One Siberian lake holds a fifth of all the unfrozen fresh water on Earth

Lake Baikal is both the world's oldest lake, at roughly 25 million years, and its deepest, plunging to about 1,700 metres. Its sheer volume means it holds around 20% of all unfrozen fresh water on the planet's surface, and its age has let a uniquely isolated ecosystem evolve inside it found nowhere else.