Diana Nyad is swimming her way back to America in a new attempt to become the first person to swim to Florida from Cuba without a shark cage.
Nyad, 62, started the 103-mile journey late Saturday night from Havana, Cuba, where she encountered box jellyfishes and has been stung at least four times already.
"Diana is swimming backstroke right now leading with the cap-covered part of her head to minimize contact. There are so many jellyfish..." a member of Nyad's team posted to her Twitter account.
Observer Steve Munatones said on Nyad's official blog that it will be a long, tough journey for the endurance swimmer.
"If this swim is the equivalent of five English Channels, and I think it is, in terms of time, she's just swum one English Channel, 25 percent of it backstroke," he wrote.
Nyad is still swimming at her regular pace of 50 stokes per minute.
"Today is more like swimming. I don't know what you would call last night ... probably surviving," Nyad said according to her blog.
Nyad ended her last attempt in September 2011 after more than 40 hours and 67 nautical miles of swimming, and two Portuguese Man-of-War stings.
"The medical team said I should not go another two nights in the water and risk additional likely Man-of-War stings which could have a long term cumulative effect on my body. But for each of us, isn't life about determining your own finish line? This journey has always been about reaching your own other shore no matter what it is, and that dream continues," Nyad called out to her flotilla of four escort boats from the water, according to her website at the time.
If she had completed that swim, she would have broken her 1979 record, when she swam 102.5 miles from the Bahamas to Florida.
ABC News' Enjoli Francis, Eric Noll, ABC News Radio and The Associated Press contributed to this story.