Underwater Search for Famous Russian Free Diver Natalia Molchanova Called Off

— -- Spanish rescuers have ended their underwater search for Natalia Molchanova, a world-renowned Russian free diver who disappeared during a recreational swim this weekend, but will continue looking for her on the water's surface.

Galina Zveryaeva of AIDA, the international diving federation, said that effort "will continue for several days."

Molchanova, a 23-time world champion holding 41 different records under her belt, was regarded by many as one of the greatest ever in the sport of free-diving.

"She was a free-diving superstar,” Kimmo Lahtinen, president of AIDA, told The New York Times.

Molchanova, of Moscow, was capable of holding her breath for 9 minutes and diving as deep as 300 feet.

According to a statement released by AIDA, the 53-year-old Russian was free-diving off the coast of Spain Sunday when she was reportedly separated from her peers. AIDA said Molchanova likely "got into [a] strong underwater current."

Her son Alexey Molchanova told the New York Times that after two days of searching, he did not think she'd be found alive.

"It seems she'll stay in the sea," Alexey Molchanova said. "I think she would like that."

While increasingly popular, free-diving is also considered one of the deadliest extreme sports. Divers often have to be helped the surface and revived because diving to such depths squeezes the lungs.

At 300 feet, the lungs go from the size of a football to the size of a tennis ball. The physical strain can be so great that divers black out before reaching the surface.

In November 2013, free diver Nick Mevoli, 32, of New York, died as he tried to dive nearly 230 feet without fins during a competition in the Bahamas. In May, he'd plunged 328 feet on a single gulp of air and became the first American to break the 100-meter mark.

ABC News' Matt Gutman and Freda Kahen-Kashi contributed to this story.