Athletes and eco-activists swam across the Dead Sea on Tuesday, the first people to thrash their way over a body of water so salty that drinking from it is tantamount to poisoning.

The swimmers crossed from Jordan to Israel to raise awareness of what they said was an environmental disaster that has shrunk the Dead Sea’s surface by one third of its size over the last 30 years. Environmental group EcoPeace Middle East, one of the organizers of the 15-km swim, said it had receded by about 25 metres over the above-mentioned period.

The swimmers wore snorkels and face masks to stop the water—around 10 times saltier than a regular sea—from touching their eyes or entering their lungs during the seven-hour crawl.

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A medical team accompanied the 28 swimmers, because ingestion of Dead Sea water can be fatal if not treated immediately.

“This was unlike anything I’ve ever done,” said Kim Chambers, 39, a renowned open-water swimmer from New Zealand.

She said the few drops of water that touched her eyes felt like acid. Additionally, crossing through such extremely salty and buoyant water made it challenging to swim.

“The swim took incredible teamwork. We had unprecedented diplomatic support from Israel and Jordan to make it happen. That’s what’s needed to bring attention to an issue that needs attention right now,” she said.

The sea, which is mentioned in the Bible, sits at the lowest point on Earth.

EcoPeace middle East blames Israeli and Jordanian mining, creating evaporation ponds from which minerals are extracted, and the diversion by Israel, Jordan and Syria of Jordan River water that flows into the lake.

EcoPeace Middle East, whose membership includes Israelis, Jordanians and Palestinian Authority, said the event was aimed at highlighting the Dead Sea’s plight and urge government action to save the natural wonder, which is also a popular tourist site.

“We see the life-threatening challenge of the swim as parallel to the challenges facing the Dead Sea,” Gidon Bromberg, Israeli director of EcoPeace Middle East, said in a statement.

The Dead Sea, about 425 metres below sea level, is bordered by Israel and Jordan.

Many visitors come for the therapeutic properties associated with its mineral-rich waters and mud, and resort hotels have been built along the Israeli and Jordanian shores to accommodate them.