The freediver strapped on fins and plunged into the warm Indian Ocean off the coast of Madagascar. Sperm whales, each the size of a school bus, circled him. Here were some of the world’s largest predators, with mouths full of teeth 8 inches long. The water rippled and his chest vibrated as they echolocated, a clicking form of language. What were they saying?

The man in water, Fabrice Schnöller, was just a regular lumber store owner from a remote French island. Fast forward six months, he’d transformed himself into a DIY version of Jacques Cousteau, the renowned ocean explorer. With a homemade contraption fashioned out of water-proof video cameras and microphones, Schnöller watched and listened to sperm whales in their natural habitat.

Today, seven year later, Schnöller holds the largest repository of sperm whale recordings. He and his team lack scientific know-how or formal degrees, but they’ve amassed a pool of data so rare that university researchers now collaborate with them.

How did this happen? And what have they discovered?

Schnöller’s story and others are told in a new book, DEEP: Freediving, Renegade Science, and What the Ocean Tells Us about Ourselves, by author James Nestor.

Nestor, a Tustin native and graduate of Foothill High School, will recount the adventures of these self-made, free-diving explorers at a public lecture at 4 and 7 p.m. today.

Nestor said he encountered Schnöller after having “his mind blown” while covering a freediving championship in Greece in 2011. Competitors held their breath four minutes while plunging to a depth of 300 feet, he said.

“I thought it was the most insane thing I’d ever seen,” said Nestor, who has lived the past 20 or so years in San Francisco. “And I thought what a waste they were only using it for competition.”

Then he found freedivers who are plumbing the ocean’s mysteries.

The freediving citizen-scientists he chronicles explore sea life, our own capabilities – and the intersection between the two.

“When you freedive oftentimes dolphins welcome you into their pods,” said Nestor. That access allows these explorers to interact with sea life in ways that others sometimes cannot.

“You can’t do that with scuba. You can’t do that with robots,” he said. “These guys are doing that and discovering some extraordinary things.”

He said they’ve discovered that dolphins have first names and last names. Yes, really, he said. “They communicate through whistles and each dolphin has a unique one,” Nestor said.

In a phone interview Tuesday, Nestor recalled the pleasure of swimming at Corona del Mar State Beach. It’s one of his earliest childhood memories.

He said we test our boundaries in the ocean. We explore the intersections between human and marine life.

“We share the same mammalian dive reflexes as whales and dolphins,” he said. The more we understand about dolphins and other mammals, he said, the more we may see ties emerge.

“We’re just now beginning to explore our potential in the water.”

Contact the writer: nshine@ocregister.com or Twitter:@nicolekshine