Growing cities, overuse of fertilizers and factory wastewater have degraded China's water supplies to the extent that half the nation's rivers and lakes are severely polluted. China aims to spend $850 billion to improve filthy water supplies over the next decade, but even such huge outlays may do little to reverse damage caused by decades of pollution and overuse in Beijing's push for rapid economic growth. Photo: Fishermen row a boat in the algae-filled Chaohu Lake in Hefei, Anhui province, June 19, 2009. (Photo by Jianan Yu/Reuters)

A young anoa, also known as midget buffalo, named Tycoon runs around at the Zoo in Berlin, Germany, on November 9, 2012. (Photo by Maurizio Gambarini/EPA)

Riot police runs as a woman falls down during a May Day rally in the Kreuzberg district of Berlin, Germany, 01 May 2016. (Photo by Kay Nietfeld/EPA)

Peter Dinklage, left, and Lena Headey attend the 67th Primetime Emmy Awards on Sunday, September 20, 2015, at the Microsoft Theater in Los Angeles. (Photo by Alex Berliner/Invision for the Television Academy/AP Images)

Fallen leaves cover the undergrowth as a commuter on a bicycle makes her way through the Tiergarten park in central Berlin on December 7, 2020. (Photo by Odd Andersen/AFP Photo)

Sina prepares for the reopening of the latex fashion shop Savage Wear as the coronavirus disease (COVID-19) lockdown measures are eased in Berlin, Germany, March 9, 2021. (Photo by Hannibal Hanschke/Reuters)

Located at the foot of the Rocky Mountains on the North Saskatchewan River, the rare phenomenon occurs each winter in the man-made lake. Photo: Ice bubbles in Abraham Lake. (Photo by Chip Phillips/Rex Features)

Lake Baikal is the world's oldest lake, at 25 million years (possibly older), and deepest, averaging 744.4 metres (2,442 ft).

Located in the south of the Russian region of Siberia, between Irkutsk Oblast to the northwest and the Buryat Republic to the southeast, it is the most voluminous freshwater lake in the world, containing roughly 20% of the world's unfrozen surface fresh water.