(CNN)Wearing nothing but a bathing suit, a swimming cap and an underwater mask, the 40-year-old woman plunged into a carved-out section of a frozen Siberian lake, before diving under the ice to swim in water estimated to be around 32 degrees Fahrenheit.
Russian woman who swam under Siberia's ice may have broken the world record
Footage tweeted by the English language Siberian Times shows the 40-year-old woman from Moscow entering the carved-out section of a frozen Lake Baikal, before she started the underwater ice swim.
Yekaterina Nekrasova, who took up free diving four years ago, then held her breath for a minute and a half as she covered the 85 meters (279 feet) of a frozen Lake Baikal on January 7 -- the Russian Orthodox Christmas Day.
She is believed to have set a world record with her attempt. A spokeswoman for Guinness World Records told CNN they have received details of Nekrasova's attempt but have yet to verify the landmark swim.
Footage filmed from above the surface shows members of her support team following behind in wet suits, in case of emergency. According to the Siberian Times, holes were cut in the 10-inch-thick ice at regular intervals in case she needed to abort the swim.
The challenge was filmed from both above and beneath the surface. Nekrasova can be seen descending a ladder, then following a route marked by a cable for a minute and a half. At the end she exits the water by climbing up another ladder.
Met by her support team, Nekrasova emerges to say in English: "I'm OK."
Lake Baikal holds several global records itself. Somewhere between 20 and 25 million years old, it is the oldest existing freshwater lake on Earth. Reaching down as far as 5,315 feet, it is the deepest continental body of water, as well as being the world's largest freshwater lake by volume -- it holds about one-fifth of the fresh water on Earth's surface, some 5,500 cubic miles.
Posting on Russian social network site VK, Nekrasova said the original plan was to swim on January 6 but "extreme weather" -- including a "very strong frost" and stormy winds -- delayed it.
While she knew that she could "comfortably" swim 75 meters (246 feet), Nekrasova said doubts began to creep in.
"I thought what if I would freeze before the start, or the mask would freeze or fog up, or I would stick to the ice at the finish line. And of course I didn't know how long I could dive in a new place," she wrote.
The air temperature was as low as -22 degrees Fahrenheit but felt more like -43.6 on January 6, she said. Conditions were "dangerous and dark under the ice," which convinced the team to postpone the attempt.
Nekrasova described what happened the following day as a "Christmas miracle."
"The weather warmed up to -21 (degrees Celsius, -5.8 Fahrenheit ), the wind slightly moderated," she wrote. As her support team prepared the site with safety lanes and holes in the ice, she remained at her hotel.