Annaleise Carr has started a 75-kilometre trek — one and a half times her last feat — at Presque Ile State Park in Erie, Penn.
The record-breaking swim, a fundraiser to send kids with cancer to camp, will end in Port Dover, Ont., and is estimated to take about 30 hours.
Camp Trillium and Annaleise's supporters tweeted early today that the 16-year-old swimmer began her voyage at 5:21 a.m. in calm conditions.
Members of Carr's support team said they were hoping for good weather since poor conditions could seriously hinder her efforts.
The start time has been moved up in an attempt to avoid possible thunderstorms Saturday afternoon.
Her team says the 16-year-old has been swimming six to eight hours a day to prepare, was feeling confident and couldn't wait to get started.
"She's such a calm girl. Before her last swim, she was just as calm before she went into Lake Ontario," said Mark Ghesquiere, a physician who will follow her during the swim.
"Now she's two years older, more mature, she's better trained than she was then because she's got that much more strength to her, so I think even though it's a longer swim, if anything, she's going to do very well."
Carr's 2012 swim raised $115,000 for Camp Trillium.