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Travel writers often describe beautiful places as having “pristine”, “pure”, “crystal clear” waters. But which bodies of water are truly the cleanest and the clearest? Where in the world do breath-taking aesthetics reflect the science of “pristine”?

Both water quality and water clarity are important technical considerations for environmental scientists. Impacts on the quality and clarity of a body of water can come from its source, its depth, the sun, natural phenomena (ranging from weather incidents to volcanic activity) and the living organisms in and around it (people included).

Clean and clear waters do exist throughout the world, if you know where to look. The following are some of the most pristine bodies of water in the world.

Crater Lake, United States
The 1,943ft-deep Crater Lake, located in Oregon, was formed 7,700 years ago when the volcano Mount Mazama collapsed after a huge eruption, and according to the US National Park Service, it may be the cleanest large body of water in the world. Crater Lake is so pristine because it is fed almost entirely by snow and rain; there no rivers or streams flowing into it bringing nutrients, sediment and/or contaminants. In addition, since Mazama is dormant today, no gases or liquids affect the lake water, as is sometimes the case with other volcanic lakes.

The lake’s vibrant colour is a strong indicator of its clarity. “Crater Lake is this amazing blue that you really have to see to believe – it’s hard to even capture it in a photograph,” said park ranger Dave Grimes. It appears this way to our eyes because plain water molecules absorb the longer colour wavelengths of green, yellow, orange and red and bounce back the shorter blue wavelengths, Grimes described. If there were more particles in the lake, or less water, it would appear less blue.

Lake Baikal, Siberia
In southeastern Siberia, north of the Mongolian border and about 70m from the city of Irkutsk,  Lake Baikal holds one-fifth of our planet’s fresh water. Baikal is the deepest lake in the world, its waters plunging down about 1,700m.

“A pattern you see all over the world”, explained Darran Crabtree, director of conservation for the central and western New York chapter of The Nature Conservancy, “is that where you do have very deep bodies of water relative to the input area [the watershed], you have a real fighting chance for having high water quality and clarity.” Although Lake Baikal’s watershed – the area of land served by a body of water – is fairly large, the lake has such incredible volume that most pollutants get diluted deep into its waters.

Lake Vostok, Antarctica
In the historic culmination of a 30-year mission, Russian scientists finally successfully penetrated a 20-million-year-old subglacial lake in Antarctica earlier this year. Hidden under a sheet of ice that was two-and-a-half miles thick, Lake Vostok had previously been untouched by the outside world, causing researchers at the Russian Arctic and Antarctic Research Institute to predict that it may be “the only giant super-clean water system on the planet”. The largest in a network of more than 200 subglacial lakes, Lake Vostok covers about 15,540sqkm –  nearly the area of North America’s Lake Ontario –  and reaches a depth of 1,000m, making it the third deepest lake in the world. (Lake Baikal is the deepest and Lake Tanganyika in East Africa is the second deepest.)

A patch of the South Pacific Ocean
Fewer organisms and less organic matter generally leads to clearer waters. Scientists from the University of the Mediterranean’s Oceanography Centre of Marseille discovered that a patch of ocean near Easter Island in the South Pacific Ocean is one of the most lifeless systems on the planet. With waters as clear as some of the purest fresh water, the researchers concluded in their study, it may be the clearest section of ocean on earth.

Lake Malawi, East Africa
From a life-poor patch of ocean in the South Pacific to a life-rich lake in the East African Rift System, clear waters can be found in all different corners of the world. Lake Malawi, also called Lake Nyasa, is the second deepest body of freshwater in Africa and one of the deepest in the world, reaching a depth of 706 meters. Serving Malawi, Tanzania and Mozambique, the 579km-long lake is accessible in Malawi via the towns of Mangochi or Monkey Bay in the south and via Mzuzu, Karonga or Nkhata Bay in the north.

Lake Malawi is classified as a Unesco World Heritage site due to its diversity of endemic fish. Providing habitat to more fish species than any other lake in the world, its cichlids fish are considered as important for the study of evolution as Charles Darwin’s finches in Ecuador’s Galapagos Islands. Its clarity remains relatively high, though, largely because of its depth. These conditions make Lake Malawi an extraordinary place for freshwater scuba diving. In the lake, however, there is the risk of bilharzia, an infection by a type of parasitic worm, so travellers who choose to swim or dive there often stock up on the treatment, Praziquantel tablets.

Blue Lake, New Zealand
A 2011 study by the Auckland-based National Institute of Water and Atmospheric Research found some of the clearest waters on record in the small Blue Lake, part of Nelson Lakes National Park in New Zealand’s Southern Alps, accessible from the village of Saint Arnaud. While relatively shallow (only seven meters deep), Blue Lake’s horizontal visibility –  the measured length one can see underwater –  can get up to about 80m. The lake is spring-fed by neighbouring Lake Constance (a glacial lake), but most of the water’s particles are filtered out when it passes a dam between the two lakes formed by landslide debris, scientists from the institute found.

Hornindalsvatn Lake, Norway
Although France’s Lake Annecy has gained the reputation of being “Europe’s cleanest lake”, Norway is home to freshwater bodies that are far deeper and less visited. Located near the town of Grodas, Hornindalsvatn Lake, the deepest lake in Europe at about 514m, is surrounded by beautiful mountains, glaciers and fjords. Snowmelt is responsible for much of the lake’s water, while run-off streams from glaciers do not drain directly into the lake, according to Norway’s tourism board, resulting in high levels of quality and clarity.

Travelwise is a BBC Travel column that goes behind the travel stories to answer common questions, satisfy uncommon curiosities and uncover some of the mystery surrounding travel. If you have a burning travel question, contact Travelwise.

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