Lewis Pugh is no stranger to cold water.

The 50-year-old from Plymouth, England, is one of the world’s most celebrated endurance swimmers and has been jumping into frigid lakes and oceans for close to two decades with nothing more than a Speedo, goggles and a swim cap in an effort to draw attention to the most vulnerable ecosystems on Earth.

On Thursday in East Antarctica, he is attempting what he describes as his toughest swim to date — a 1 kilometer (about 0.6 miles) swim across a supraglacial lake. If he makes it across, he’ll be the first human to swim one of these lakes, which form when meltwater from a glacier collects on the surface of an ice sheet.

“I’ve seen the oceans change, it’s taken change on my watch,” Pugh said. “As the United Nations ambassador for the oceans, I take it personally, I don’t want to stand by when the ocean is being trashed and being disrespected and changing. I want to stand there and defend it and protect it.”

Pugh said he will face extreme temperatures during his swim. The water will be just above freezing, and strong winds across the Antarctic ice sheet could push the air temperature to minus 37 degrees Celsius (about minus 35 degrees Fahrenheit) or colder. During the swim, he’ll be surrounded by a team to ensure his safety.

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Pugh, who has helped advocate for in environmental advocacy for more than 15 years, worked with glaciologists from Durham University in England to map out one of the lakes in a region known as Dronning Maud Land. By swimming across a supraglacial lake, Pugh and other ocean advocates hope to draw enough attention to East Antarctica's landscape that world leaders will take action to protect it.

A study led by the team at Durham University that worked with Pugh discovered that more than 65,000 of these supraglacial lakes had formed on top of the East Antarctic ice sheet in January 2017 during the summer melt season. The study was the first of its kind using new, higher-resolution satellite imagery to map and count lakes in greater detail.

“We’ve known for some time that lakes are forming in East Antarctica, but we were surprised at quite how many had formed and all around the ice sheet margin,” Chris Stokes, a professor at Durham University and the lead author of the study, said in an email.

Scientists and researchers have only recently started studying the formation of these lakes in East Antarctica. The next step in their research is to examine how much the number of lakes varies from year to year.

Lewis Pugh trains in a river under the Antarctic ice sheet, as he prepares for "toughest" swim of his life on Jan. 20, 2020. Kelvin Trautman

“Lakes themselves are not an indication of a warming climate and they have always been around,” Stokes wrote. “However, we would expect more and more lakes to form in a warming climate, and this is exactly what we have seen in Greenland, more and more lakes forming and also forming at higher elevations. We don’t really have those data yet for East Antarctica, but it is something we are working on.”

Antarctic is governed through a treaty system that ensures the continent is used only for peaceful purposes and is open for scientists. Under international law, 25 nations plus the European Union need to agree on any measures to protect the continent, but an effort to turn part of the Antarctic ocean into the world’s biggest nature sanctuary has been stalled. China and Russia are the lone holdouts.

After the swim, Pugh will travel to Moscow to meet with Russian leaders in hopes of convincing the country to support the marine protection effort, which will ban all fishing in the area.

Pugh along with other influential figures in the business, sports and political worlds launched Antarctic 2020 in 2018 as a mission to protect more than 7 million square kilometers (about 2.7 million square miles) of the Southern Ocean.

“I'm urging world leaders to be courageous to take the important hard decisions which they have to take in order to protect the environment,” Pugh said of their ultimate goal to create a marine protected area in East Antarctica. “Allowing this area to recover and restore itself, that’s the dream.”