The prince is almost ready. A metal harness carrying two high-pressure air cylinders is eased onto his back. He tightens its straps around his waist. He checks his breathing regulator by inhaling and exhaling through its mouthpiece. He cleans off his face mask. His practised way with these procedures confirms his numerous dives in temperate waters.

He adjusts the row of lead weights on a belt around his waist and looks at me. “This weight-belt is very heavy,” he says.

“The belt weighs 50 pounds,” I tell him. “The first 25 pounds is to overcome the buoyancy of your inflatable suit. The second 25 pounds is to overcome reluctance.” He responds with a half-smile.

He puts the mouthpiece of his breathing regulator between his teeth, inhales, pushes off from the insulated floor and slides into the water. After bobbing for a few seconds in the ice crystals, he presses the exhaust button in his suit and sinks. The neoprene glove on his right hand has a firm grip on the yellow line hanging in the dive hole. He will wait for me beneath the ice.

I slip into the water and check my breathing regulator. A member of the Scotland Yard protection team looks at me with eyes that have squinted the length of a gun barrel.

I exhale and descend into the chimney of ice. It took most of an afternoon to cut the hole and remove two tonnes of frozen water. The sides of the chimney have the patina of old ivory.

I ease my way down the chimney, beneath the ice, and come face to face with the prince in crystal clear water. I’m hyper-vigilant for a multitude of signs that will keep my partner whole and bring him safely back to the surface. In this airless, voiceless world, I focus on three things: breathing rate, eye movement, body language. His breathing rate is paced. His eyes are locked on mine. His fins move easily in the water beneath him.

This is my homeland, a place I regard with deep suspicion. The ceiling of ice splaying out to invisible horizons. A tangle of kelp covering the sea floor. Water so cold it kills an unprotected diver in minutes. Five years before, I led the first of four expeditions to this bay. Our objective was to develop techniques to allow scientists to work safely beneath the ice. Supported by the Canadian government, we made more than 800 dives in this cove and other polar regions. Last year, not far from where we’re swimming, we constructed Sub-Igloo, the first staffed polar station. The station was a transparent acrylic sphere, filled with air, that was big enough to accommodate three divers on a circular bench. Its entry and exit hatch was a small, round opening in the floor. Air pressure inside the sphere kept the ocean at bay. Sixteen struts connected the sphere to ballast trays. For six weeks, Sub-Igloo was our refuge and science centre.

A diver appears out of the blackness behind the prince. It’s Rick Mason, one of the most trusted members of our team, and he’s carrying a 35mm camera with a strobe light.

The prince struggles to control the air in his suit. He adds too much and his head bumps hard against the ice. He presses the exhaust button and sinks too quickly. His steady breathing confirms that he’s unflustered as he wrestles with the unalterable principles of buoyancy.

I scan the dimensions of our claustrophobic realm. The undersurface of the ice looks like a storm cloud restraining mercury-like pools of our exhaust air. The vertical vault of water distorts the near and far of things. Once the prince has his buoyancy under control, I motion for him to follow. Slowly, we head west, keeping the ice above our heads. I point down to the shadowy piles of steel ballast that once held Sub-Igloo to the sea floor.

A few minutes later, we encounter the first ice stalactite, a hollow tapering tube as long as a forearm, hanging from the ice. It encloses a plume of descending brine filled with a community of life including diatoms and algae. The most visible creatures are tiny, brown crustaceans called amphipods. The prince looks at the amphipods for a long time and gazes up at the ceiling of ice. Later, he tells me that “every mountain and river has an almost sacred character of its own.”

We continue west until we encounter a ridge of shore-fast ice. In the feeble twilight, we stare at broken blocks lying at harsh angles. It is a starkly forbidding place of fissures and corrugations. The prince is looking at ice as cold and malevolent as beauty can be.

I check my watch. Twenty-five minutes have elapsed; it’s time to swim back to the dive hole. In the celestial light, a lion’s mane jellyfish trails a skein of stinging tentacles. I turn a slow, 360-degree circle to see if anything is watching. So far, we’ve avoided the evil luck of a Greenland shark. They live in deep water but visit the shallows. Slow-moving carnivores, they reach lengths of six metres. A few have been found with caribou and human bones in their stomachs. The fact that we’ve not seen them in this bay has never been comforting.

ALMOST A DECADE LATER, in July, 1984, I find myself in Kensington Palace, London, in the office of the personal assistant to the Prince of Wales.

The lieutenant-colonel asks, “Are you ready? He’ll be here in five minutes.”

It has taken a series of interlocking events to be here. A five-year search for a 19th-century British shipwreck 90 kilometres east of where the prince made his 1975 dive. The discovery of the Breadalbane in 100 metres of water. The construction of a 20-person base camp that allowed us to make 10 dives to film the 1853 wreck. An almost-completed documentary about the project, The Land That Devours Ships, directed by Bill Mason for the National Film Board.

Six months ago, I sent a letter to the prince asking if he might consider introducing the film. In a letter from Buckingham Palace, his private secretary wrote back: “His Royal Highness would be happy to contribute to your film. It would be helpful if you could let us have a draft of what the Prince of Wales might say. His Royal Highness would then like to see the film before deciding on the form of his introduction.”

I check to make sure the lights are in place, the sound equipment tested. The film’s director, Bill Mason, gives me an affirmative nod. “We’re ready,” I say.

The door opens, and the prince enters the room. He’s wearing a double-breasted grey suit and smiling. We exchange greetings, and I introduce him to the members of our four-person documentary film team.

“I’m sorry, I’ve only got 40 minutes,” he says, “so we should get started.”

Standing behind a model of a 19th-century three-masted ship, he faces the camera. “I’m ready,” he says. “Let’s see if I have the talent to make this work.”

“Camera rolling,” says the cinematographer. The prince begins. “More than a century ago, ships like this one sailed out from England and into the Canadian Arctic in search of the Northwest Passage. Having served in the Royal Navy myself for a few years, I think I can imagine, just a little bit, about what it must have been like to be on board a ship like this, crossing the cold North Atlantic in those terrible stormy seas. And fighting one’s way through those ice-choked islands.”

His voice is warm and humble. “A few years ago, I was lucky enough to visit the Canadian Arctic and to dive under the ice with Dr. Joe MacInnis. That was one of the most interesting and exciting things I have done for a considerable time. It was that particular experience that helped me to discover why so many of those ships never came back from the great expeditions of the 19th century. It was the loss of two of those ships, the Erebus and Terror, under the command of Sir John Franklin, that launched the greatest search in maritime history. And it’s a search that’s continuing even today.”