Imagine slipping through a small hole in a vast expanse of thick Antarctic sea ice and descending down, down, down, into a primeval world that you may be the first human ever to visit. It’s cold, somewhere between minus 18 degrees Celsius and 10 degrees Celsius. You can feel it, even through your protective dry suit. Little light penetrates the icy lid above you.

Far from being the tomb-like silent place you had thought, beyond the slow in-out of your breathing you can hear snow tractors rumbling across the sea ice above, and around you, sleek Weddell seals are singing their otherworldly song.

For underwater photographer Bill Curtsinger, this is heaven on earth.

“I would pack my bags tomorrow if I had a chance to go back,” Curtsinger said in a recent phone interview.

Curtsinger has made four trips to Antarctica, and has made a name for himself as a preeminent underwater photographer. His nature photos of life in some of the harshest environments on earth have graced the cover of National Geographic six times, including the classic closeup of a cuddly harp seal pup (Jan. 1976), a baby black green turtle swimming out to sea for the first time (Feb. 1994) and a close-up of a gray reef shark (Jan. 1995).

In addition to the cover stories he has photographed and written close to three dozen stories for National Geographic. Other stories and photos have appeared in a wide range of international publications, including Smithsonian, Paris Match and BBC Wildlife.

He has previously published six books of his nature photography, and is currently touring to promote his most recent book, “Extreme Nature: Images from the World’s Edge.”

Curtsinger makes an appearance April 13 at IslandWood on Bainbridge Island, in partnership with Eagle Harbor Books.

“Extreme Nature” is a collection of his finest work from locations around the world, but it starts in his favorite spot, Antarctica.

As he describes it in the book, with its crystal clear underwater visibility, “It is like diving in a glass of ice-cold gin, and for me, an intoxicating experience of the best kind.”

In four trips spanning four decades, the photographer has spent more than 18 months on and under the continent at the bottom of the world.

“The Antarctic environment can be cruel and unforgiving: where many species struggle mightily to stay alive,” he writes. “It’s also a place of extraordinary beauty and biological diversity, where the resident flora and fauna has evolved interesting strategies for life in a freezer.”

Curtsinger said his goal in writing the book was to share his passion for the natural world.

“I hope that people will appreciate it for the things they’ll never see in their lives, especially the polar world.”

He calls these “tough times” for the environment. “I want my photos to make people see that.”

To that end, his photographs are as much about the emotions he experiences as the animals he captures on film.

“These photos are about feelings. I didn’t want to just describe them in a scientific way,” he said.

In describing his groundbreaking photos of graceful emperor penguins swimming underwater in McMurdo Sound he writes, “Their performance had a beautiful circus-like quality, with me at center stage. They swam above, below, and all around me showing off their superior swimming skills, while I watched and photographed in awe.”

He calls the ethereal Antarctic jelly fish “Beautiful Drifters,” and said “These are the little stars and planets of my own little dreamscape. These animals look to me like organic space stations, galactic travelers, suspended in some galaxy far, far away.”

Even the toothy shots of circling gray reef sharks evoke for the viewer the sense of wonder that Curtsinger has never lost, in all his travels.

The book also chronicles a few travails along the way, including an attack by those reef sharks while snorkeling in Micronesia. Curtsinger explains that gray reef sharks are territorial, and their attacks have more to do with defending that territory than looking for a meal. Still, a shark attack is never a good thing.

As he snorkeled in the tropical waters, he saw a “blur” coming at him from below. The shark went for his head, and as he threw up his left hand for protection the shark took a bite.

“I don’t remember any pain associated with the bite itself. What I remember most was the force of the impact. It felt like a torpedo had hit me, so powerful and strong that it took me a few seconds to recover,” he writes.

As he swam for the boat the shark struck again, raking his shoulder with its teeth. At that point his life began flashing before his eyes, from a childhood dog, to his girlfriend to his National Geographic boss.

“I remember feeling bad that I wasn’t going to be able to finish this assignment.”

He did of course complete that assignment and many more.

Curtsinger shoots all his photos with film, which adds to the challenge of underwater photography — you can’t change film underwater. Sometimes he is able to take along an underwater crew with multiple cameras, other times he just has one camera, 36 frames.

In spite of the bone-chilling cold Curtsinger noted, “I never felt the cold till I ran out of film.”

Bill Curtsinger will speak about his book and life as an underwater photographer 7:30 p.m. April 13 at IslandWood, 4450 Blakely Ave. NE, Bainbridge Island. The event is free, but advance registration is required. Register online at www.islandwood.org or by phone at (206) 855-4300.