The search for answers began with a single leaf of paper, rolled up on the ocean floor, 540 feet beneath the swells. The paper was lifted from the darkness by the lights of a remotely operated vehicle dropped over the side of a supply ship called the Atlantic Osprey. The men who piloted the ROV — from inside a quiet, windowless container that had been welded to the ship's deck — trained its cameras on the paper. They were working in black and white, more out of habit than anything else. When they did what they usually did, maintaining the underworks of oil platforms, color rarely factored into it; everything down there was machined from the same shade of gray. But now these men were doing different work, and they leaned into their monitors and tried to make out the black type on the white paper. They were able to read just a few words about how to fly a helicopter. "That's when we knew we were on the right track," Allan Chaulk says.

On the table in front of him, inside the Transportation Safety Board of Canada's engineering branch in Ottawa, Chaulk has rolled up a piece of paper into a cylinder smaller than a soda can. "About like that," he says. The impossible image of the paper on the vast ocean floor, anchored by silt and tiny pebbles, now occupies a permanent space in his memory bank. He can close his eyes and cover them with his hand, which he does often, and see the paper in front of him again.

Chaulk had expected to be on a different ship that weekend. He had planned to take a few days off from the TSB's Atlantic office in Dartmouth, Nova Scotia, where he is one of the resident interpreters of aircraft wreckage. Chaulk, a native of Corner Brook, Newfoundland, was waiting for the ferry back home with his son, Kevin, and their snowmachines. They were going to ride the trails with some of the boys. That was before his cell phone rang. It was Mike Cunningham, Chaulk's boss at the TSB. "I might need you," Cunningham said.

A few minutes earlier that mo...