ERIE, Pa. - A teen girl believed to be the youngest to swim across Lake Ontario two years ago is now taking on an even bigger challenge: a marathon swim across Lake Erie.

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.