The woman billed as the world's greatest freediver is missing and presumed dead after failing to resurface from a dive off the coast of Spain.
Natalia Molchanova holds 41 world records and is a 23-time world champion in freediving. The sport requires divers to hold their breath rather than use scuba gear or oxygen tanks.
AIDA, the international freediving association, said that the 53-year-old Molchanova was "recreationally freediving" without fins on Sunday when she became separated from her group. She disappeared approximately two miles northwest of the port of La Savina at Poniente de es Freus.
"Search efforts have been ongoing," the association said in a statement.
AIDA said Molchanova's passion for the sport "burned so deep" that she had dedicated her life to freediving.
"She was an inspiration to all freedivers and despite being one of the fiercest competitors in the world, she was always calm and relaxed during competitions," the statement said. "The cause of Natalia's disappearance is unknown, but she was doing what she loved."
Molchanova — who could hold her breath for nine minutes and dive to a depth of more than 330 feet — had written freediving handbooks and trained thousands in the sport.
The Russian Freediving Federation — which Molchanova headed out of Moscow — called Molchanova's disappearance a "tragedy" and made clear it was unlikely she would be found alive.
“The search is still officially ongoing, but, to our deepest regret, the probability of success is small,” it said in a statement posted on the federation's website.
Kimmo Lahtinen, president of AIDA international, told NBC News that Molchanova was a "freediving superstar."
"Superstars will not die," Lahtinen said. "They will shine still after they are gone."
Fellow freediver William Trubridge was among those offering tributes to Molchanova following news of her disappearance.