Natalia Molchanova, widely regarded as the greatest free diver in history, wore her favorite purple wet suit Sunday on a chartered boat off the coast of Formentera, an island near Ibiza. Only 1.5 millimeters thick, the wet suit was low buoyancy, designed specifically for divers like her who want to go deep on a single breath. It was a Molchanova, the brand she started with her son, Alexey Molchanov.

With clear skies and a light breeze on the Balearic Sea, off the eastern coast of Spain, Ms. Molchanova attached a neck weight — a pound or two, at most — that would help her descend. Then she dipped into the water to begin what divers call breathing up. A lower heart rate enables divers to use precious oxygen more efficiently. To relax deeply, Ms. Molchanova would inhale for a few seconds, then exhale twice as long and twice as deep.

Those may have been among Ms. Molchanova’s last breaths. She then proceeded to attempt a recreational dive to a depth that was modest for her, and she never resurfaced. After two days of intense search efforts, Alexey Molchanov said Tuesday afternoon that his mother, 53, was not expected to be found alive.

“She was a free-diving superstar, and we all thought nothing could harm her,” said Kimmo Lahtinen, the president of the global federation for free diving, known as AIDA. “Nothing could happen to her, but, you know, we are playing with the ocean, and when you play with the ocean, you know who is the strongest one.”