Her death from unknown causes at her home in Solvang in California on February 6, was announced by the World Open Water Swimming Association, The New York Times reported.

Andersen, who broke 18 world marathon records, has been called the greatest female swimmer in history, according to Bruce Wigo, former President of the International Swimming Hall of Fame, which honoured her with its lifetime achievement award in 2015.

"She often beat all of the men," he told The New York Times.

After shooting to Olympic fame, Andersen, who also competed at the 1952 Olympics in Helsinki shortly after knee surgery without winning a medal, sought a way to turn her talent into a living.

In 1953, she emigrated to Long Beach in the United States to look for opportunities in open water, or marathon swimming contests, which carried prizes of about $3,000 - worth about $33,000 today.

For decades, she ran a swimming school and health spa in Los Alamito,  teaching swimming to students of all ages.

She was the first woman to complete five crossings of the English Channel and the first to win the race across it twice in a row, which she did in 1957 and 1958.

Andersen was largely successful and undefeated in her open water races, which were highly popular.

One challenge that proved too much for her, however, was an attempt to cross the Molokai Channel - a punishing route of 27 miles between the islands of Molokai and Oahu on Hawaii.

Andersen attempted it twice, in January and April of 1961.

On her second swim, which she began just after midnight, she battled sharks - she swam for a time in a specially designed cage - and was bumped by porpoises.

Andersen was swamped by 20 foot waves, and stricken with seasickness.

There were also rain squalls – but it was the current that thwarted her in the end.

Andersen fought it for nine hours until her crew pulled her from the water at 11:06pm that evening, still nearly 10 miles from Oahu.

She had been swimming for nearly 24 hours.