(CNN) Fifty-year-old Lewis Pugh says he was terrified when he plunged into the water of East Antarctica in nothing but swim briefs, a swim cap and goggles. He was even more terrified when he swam below the Antarctic ice sheet, through melting tunnels -- though he said it was the most beautiful swim he's ever done.

Pugh is known for swimming in Arctic water to raise awareness for climate change, but on January 23, he became the first person to swim in a supraglacial lake -- a lake that has formed on top of a glacier because of melting ice.

"(The swim) was terrifying for a number of reasons," Pugh told CNN. "First, the water is so cold for a swimmer. It was 0 degrees centigrade, just above freezing. But also, it illustrates very very graphically what is happening in East Antarctica."

The point he was trying to make, he said, is that it shouldn't be possible to do the swim, but it is because of cracks in the glacier.

Pugh said he was motivated by a September 2019 study in the journal Scientific Reports that discovered more than 65,000 supraglacial lakes on East Antarctica's ice sheet.

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