The most experienced freedivers manage to go no more than a few hundred feet below the surface—they play in the Epipelagic Zone. Dolphins are good to about 1,000 feet m the heart of the Mesopelagic. After that, the Bathypelagic Zone is home to electric rays, sperm whales, and a lot of unfamiliar creatures. Terms unfamiliar? Think about a bathysphere—the spherical submarine for deep exploration.

Freediving is “the most direct and intimate way to connect with the ocean.” During a three-minute freedive, “the (human) body bears only a passing resemblance to its terrestrial form and function. The ocean changes us physically, and psychically.” Unfortunately, “sometimes you don’t make it back alive.” Those words were written by James Nestor in his new book, “Deep: Freediving, Renegade Science and What the Ocean Tells Us about Ourselves.” It’s one of the best books I’ve read this year, an introduction to a part of our world that is unfamiliar to nearly everyone on the planet, and an adventure story, too.

Continuing. “At sixty feet down, we are not quite ourselves. The heart beats at half its normal rate. Blood starts racing from the extremities toward the more critical areas of the body’s core. The lungs shrink to a third of their usual size…” And, “at 250 feet, the pressure is so extreme that your lungs shrink to the size of fist…” Human freediving is a story in itself, a dangerous way to explore because, often, one emerges from the water with blood coming out of the nose and mouth, and sometimes, if the pace of re-emergence is too rapid, blackout is common and, drowning, implosion and death are distinct possibilities.

As grossly fascinating as the extreme sport may be, the magnetoreception used by sharks, who can dive below six hundred feet, is more compelling. Sharks are tuned to “the magnetic pulses of the Earth’s magnetic core.” Echolocation seems less magical—it’s the basis of sonar and sonograms—but the coded language used by sperm whales, or dolphins, as they echolocate, well, that’s something extraordinary. Going further, cetaceans (including, for example, dolphins) possess brains that differ from the human ones that we are just beginning to understand. “…dolphins…could hold two separate, simultaneous conversations with two separate modes of communication, clicks and whistles—the equivalent of a human talking on the phone while chatting online” (to which I would add: we pretend to do two things at once—texting while driving, for example, but do neither well when multitasking).

Way down in the bathypalegic zone, the author and some compadres are watching the burning sea—bioluminescence—“chemical production of light by living organisms” in a inky-black sea so far from the surface that it exists in darkness (“night never becomes day”). “The grotesque-looking anglerfish uses a little light on the top of its head to attract prey. Giant squids—which can grow more than sixty feet long…use bright flashes to communicate with other squid using something similar to Morse code.”

He becomes fascinated with bioelectricity, and explains that “every cell in your body contains an electrical charge…the electricity travels by way of a series of circuits called ion channels, tiny proteins in the membranes of cells.” Nestor goes on to discuss electric rays, animals whose design concentrates this electrical power so that it can “emit a shock of more than 22o volts.” He considers the body’s energy that the Chinese call chi, and wonders what we can learn from the ocean in order to control and maximize the use of our power.

Back, for a moment, to sperm whales, “the loudest animals on earth.” He explains, “at their maximum level of 236 decibels, (their) clicks are louder than two thousand pounds of TNT exploding two hundred feet away from you, and much louder than the space shuttle taking off from two hundred and fifty feet away. They’re so loud that they cannot be heard in air, only in water, which is dense enough to propagate such powerful noises.” They could “vibrate a human body to death.”

This is a spectacular journey, written so that the reader goes deeper and deeper into the depths of the ocean, then emerges with newfound knowledge and appreciation of earth’s wonder.

I could go on, but it’s late, and there’s not much more to say besides, “buy the book.”