Mehdi Bilel was returning home after attending a marriage in the north of Tunisia when he spotted a large lake glimmering in the hot sun, in the middle of the parched desert, about 25 km from the city of Gafsa. Mehdi Bilel stood on its shore in stunned silence. The lake wasn’t there a few days ago. At first he thought he was hallucinating. He was walking for several long hours on the road without a break, and the heat could play all sorts of tricks upon the brain. But Mehdi Bilel wasn’t imagining things. The lake had actually materialized out of nowhere.

News of the mysterious lake spread like wildfire and hundreds of Tunisians flocked to what quickly became known as the “Lac de Gafsa” or Gafsa beach to swim and cool off in the region’s 40 degree C heat. This was August 2014, and Tunisia was in the middle of a drought, which made the lake’s sudden appearance even more enigmatic.

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The origin of the formation of the lake is not clear. The most likely explanation is that a minor earthquake had ruptured the rock above the water table sending millions of cubic meters of water up to the surface. The lake covers a hectare in area and is 10 to 18 meters deep.

The mysterious lake has since become a major attraction of the area. However, swimming here is risky as the water of the lake could be contaminated with phosphates. The southern Tunisian region is rich in phosphate which is found in the region’s soil and rocks, and since its discovery in 1886, Gafsa has become the center of the country's mining industry. Phosphorous compounds are used to manufacture anything from fertilizers, pesticides, and detergents, to matches and chemical weapons. In other words, all the dangerous stuff. Besides, phosphate happens to leave behind a radioactive residue. This means, that if the lake is contaminated, the water is both radioactive and carcinogenic.

The first sign that something wasn’t quite with Lac de Gafsa appeared within a couple of days of its appearance. The color of the water changed from clear, crystalline blue to murky green due to a bloom of algae, which meant that the water was not being replenished and therefore could be teeming with bacteria and diseases.

About two weeks after the lake appeared, the Office of Public Safety in Gafsa warned the locals that it was dangerous to swim in the lake, but few heeded the warning. Throngs of people still come here to swim and stave off the desert heat.

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Sources: The Guardian / Science Alert