If you think the sea is the saltiest place on Earth, you are mostly right. Water makes up more than two-thirds of our planet's surface, and 96% of Earth's water is in the ocean. It contains thousands of billions of tonnes of dissolved salt.

The saltiness varies. Around the poles, snow and ice dilute the salt, while nearer the equator evaporation means the water is saltier.

But there are places on our planet where you can find water far saltier than the sea.

Few are more famous than the Dead Sea, nestled on the borders of Jordan and Israel. The water here is around 10 times saltier than sea water. However, it is only the fifth saltiest body of water on Earth.

The waters of Don Juan Pond in Antarctica are 44% saline

It is also not really a sea. Although the word "sea" is used quite freely, it actually denotes a large body of salt water that is partially enclosed by land. The Dead Sea is entirely land-locked, so really it is a lake. It is the saltiness that causes confusion.

The rocks at the water's edge glitter with crystallised sodium chloride where the Sun evaporates the water. It is the world's deepest hypersaline lake, reaching a depth of 1,080ft (330m), and is widely known as the lowest point on land. The shoreline is around 1,380ft (420m) below sea level, but the water level does fluctuate.

While it has been shrinking in recent years, Israeli geologists believe it will stabilise rather than completely "die". As a result, in the future the Dead Sea may resemble the world's truly saltiest body of water – which is a pond.

The waters of Don Juan Pond in Antarctica are 44% saline. It is tiny: at 4in (10cm) deep it is barely enough to paddle in.

The cause for its hyper-salinity is not entirely understood

The surrounding environment does not exactly lend itself to spa treatments either. The pond was found in the McMurdo Dry Valley, an extreme desert environment cut off by mountains where no snow falls.

Don Juan Pond was not named after the fictional libertine, but rather after the two helicopter pilots that discovered it on the notoriously inaccessible continent.

"The cause for its hyper-salinity is not entirely understood," says geologist Jay Dickson of the California Institute of Technology in Pasadena. He has spent years studying the pond, using time-lapse photography to record how it changes.

"An important factor – but not a unique property of Don Juan Pond – is that it is a closed basin," says Dickson. "An open basin is one that has water flow into it and a channel through which it drains. A closed basin doesn't have this, so any water and salt brought into the pond can't get out."

Salar de Uyuni measures more than 4,050 square miles

At this point, the water can either freeze or evaporate. "Don Juan Pond has so much salt that the temperature would have to be -53C for it to freeze, so the water evaporates, leaving the salt behind with some water," he says. "This is part of how it gets so concentrated."

While other ponds in Antarctica are fed fresh water by nearby glaciers, which melt in the summer, Don Juan Pond is not diluted in this way. Dickson says researchers are still trying to find out where the salt water comes from.

But since salinity describes salt dissolved in water, we have to move from liquids to solids to find a truly record-breaking accumulation of the mineral.

The largest salt flat in the world is found in Bolivia. Salar de Uyuni measures more than 4,050 square miles (10,500 sq km) and was formed when a prehistoric mega-lake dried up.

The basin is now paved with bloated hexagonal salt crystals that stretch as far as the eye can see. As its description suggests it is remarkably flat, varying in height by less than 3.3ft (1m).

Tourists flock to the area to admire its seemingly otherworldly appearance, while flamingos visit to breed.

Beneath the stark crystalline crust there is brine, which is rich in minerals.

Among other things, it contains half the world's supply of lithium. The Bolivian government has recently started extracting the valuable soft metal, which is used to make the batteries in our electronic gadgets.

Salt has been extracted from Salar de Uyuni for centuries. Nevertheless, the miners, who used pick-axes and shovels to dig and pile up the salt to dry, have barely scratched its surface.

Other places, encouraged by the constant demand for salt – both for food and to sprinkle on icy roads in winter – have extracted far more of the white stuff.

One of the world's biggest exporters of salt is Australia. It produces 11 million tonnes of the stuff each year, and sends 90% of it to overseas markets.

The world's single largest salt mine is in Goderich, Canada

The method used is called "solar salt production". It relies on salt-water ponds being evaporated by the fierce heat of the Sun. This industrialises the natural process at work in Bolivia, creating man-made salt flats that can be harvested.

Salt fields also contribute to China's world-beating salt production figures. According to the 2016 figures from the US Geological Survey (USGS), China produces more than 70 million tonnes per year. What they cannot harvest from above ground they extract from below, either in the form of brine pumped out, or rock salt quarried or mined with explosives.

The world's single largest salt mine is in Goderich, Canada. The mine, operated by Compass Minerals, reaches a depth of 1,800ft (549m): it is as deep as Toronto's CN Tower is tall. It stretches for 2.7 square miles (7 sq km) and produces 7.25 million tonnes of salt every year.

Its location on the edge of the Great Lakes is the key to its productivity. There is a large, ancient source of salt beneath the area, which extends under the Canadian border into the north-eastern US. It is the remains of a prehistoric sea from around 420 million years ago.

Large deposits in places like Africa and Asia may not be as well-known

There are many other such deposits. "Large hardrock [salt] deposits are known throughout the world, including the Michigan basin in the north-eastern US and south-eastern Canada, in the south-western US, in the US Gulf Coast, north-western Europe and in western Canada," says Wallace Bolen, a salt specialist at the USGS's National Minerals Information Center.

Europe also has an impressive stash of minerals. Known as the Zechstein basin, it formed during the Permian period 270-250 million years ago. The area extends from northern Britain, across the North Sea through the Netherlands, Denmark, Germany and Poland.

People in the western world have been looking for these valuable salt deposits for hundreds of years. Nevertheless, we may have overlooked some of the largest.

"Much of the attention over the many centuries has been on defining deposits in Europe and North America, so large deposits in places like Africa and Asia may not be as well-known," says Bolen.

The problem is that most information about largest salt anything is centred on mines

Perhaps unsurprisingly it is Russia, the world's largest country by land mass, that is home to the biggest "saline giants". These are vast evaporated remains of ancient seas locked underground.

A comprehensive review published in 1969 suggested that the Upper Kama Basin, west of the Urals, is home to some of the world's largest deposits of rock salt and sodium chloride brines. As far back as the 15th Century, the town of Solikamsk was established above the basin for salt-mining. It still produces potash (potassium salts) today.

Solikamsk recently made headlines after an enormous sinkhole developed due to flooding in the mine. In September 2015 the sinkhole was measured at over 394ft (120m) wide.

All in all, we know a lot about our planet's salt deposits. However, because the focus of research is often firmly tied to salt's economic value, it is difficult to find the definitively saltiest place.

Large hardrock [salt] deposits are known throughout the world

"It's hard to pin this down," says Ted Nield, editor of the Geological Society's Geoscientist magazine. "The problem is that most information about largest salt anything is centred on mines, and the largest mines may not be in the largest deposits."

For now the best we can do is to say that the saltiest place on Earth is arguably a small pond in Antarctica, or underneath a remote area of Russia. But it could also be somewhere else that nobody has yet thought to look.

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