EREBUS BAY, NUNAVUT—Small islands of crushing ice pour out of Larsen Sound, gouging trenches that criss-cross and snake along the seabed.

Like plates of the Earth’s crust slammed together in tectonic shifts, the force of butting Arctic ice floes drives the losing side down toward the sea floor or stacks floes metres thick in tall rafts of ice.

“Then you basically have a plow edge pushing forward,” says Chriss Ludin, an underwater archeology technologist, as we ply uncharted waters, close to the last position the Royal Navy’s Franklin expedition reported in an 1847 note.

The long search for Franklin’s HMS Erebus and HMS Terror began soon after the British Admiralty declared both three-masters overdue the year after that note.

In the High Arctic sleuthing for the shipwrecks today, ice forensics plays a central part.

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The scars that ice floes have cut across the seabed for millennia are clues to what may have happened in the catastrophe that killed Franklin and his 128 men, the Royal Navy’s worst disaster in Arctic exploration.

Parks Canada, lead federal agency in the search for the lost Franklin expedition, deploys its survey and diving boat Investigator from the icebreaker CCGS Sir Wilfrid Laurier each day.

The boat runs up and down lanes in search boxes drawn up in months of planning, which included discussions between Parks Canada’s underwater archeologists and Tom Zagon, a research scientist at the Canadian Ice Service.

Running parallel to Investigator, a Canadian Hydrographic Service team on a launch called Gannet, surveys with 3D sonar equipment, building a proper chart so that bigger ships can safely navigate this stretch of the Northwest Passage.

It’s too risky for the search and survey teams to push too far from the mother ship, the icebreaker Laurier.

So improving charts of the sea floor is essential to getting future expeditions closer to the point of abandonment, which was recorded in an ink note that one of Franklin’s officers sealed in a tin can.

It placed Erebus and Terror, beset by ice, at five leagues, or some 27 kilometres, north-northwest of their landfall, which was south of Victory Point.

Zagon, the ice expert, contacted the Parks Canada underwater archeologists after seeing news in 2010 that they had discovered the wreck of the Royal Navy’s HMS Investigator in Nunavut’s Mercy Bay, off the shores of Aulavik National Park.

Capt. Robert McClure abandoned the ship in 1853 when their mission to find Franklin and his men itself had to be rescued from the relentless Arctic ice.

Zagon told the archeologists that ice and its movements aren’t as unpredictable as they might seem. He had done a preliminary study of the area in 2009 just in case anyone was interested in a closer look.

He offered to do that, using images made by Canada’s RADARSAT satellites over several years, to help narrow the almost infinite possibilities down to calculated search areas. The archeologists immediately took Zagon up on his offer.

He came back with a detailed analysis that tracked where winds and currents usually carry ice floes in Victoria Strait and identified shear zones.

They are the spots where ice floes moving at different speeds slide past one another. As they grind under enormous pressure, they mark the route of the flowing ice.

Knowing that tells underwater archeologists where ice floes might have taken Erebus and Terror.

“Incredibly enough, Inuit testimony from the nineteenth century contains clues that are recognizable on spaceborne Synthetic Aperture Radar imagery,” Zagon reported.

The underwater archeologists see it as a masterful combination of modern technology with historic accounts from Inuit people who described encounters with the Franklin expedition and at least one ghost ship, drifting south on an ice floe.

“The important point is that ice information on its own is not sufficient to locate the vessels, but neither is the historical evidence,” Zagon says now. “Combining the two starts to provide some clarity.”

We aren’t far down the first lane through the western edge of Erebus Bay before an enormous ice field closes in on the horizon, roughly half a nautical mile away and drifting.

“If that comes up behind us, we may be jigged,” Ludin says, turning Investigator’s bow to veer back into an open lead.

It was time to switch to plan B, which happens often in the search for the Franklin wrecks.

Flexibility is part of the job description. The search area is constantly shifting to suit conditions.

“We’re building a block on existing coverage,” says Jonathan Moore, a senior underwater archeologist at Parks Canada. “It’s like a straight-sided amoeba.”

Sometimes in this hunt you have to adapt in seconds.

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Waters in most of the northern search area, the most promising hunting grounds for the Franklin wrecks, haven’t been surveyed and charted to modern standards. So mariners have to be cautious and think fast to stay afloat.

Even far from shore, when King William Island is just a bleak thread of land on the horizon, the ocean floor suddenly rises up. There are hidden shoals everywhere.

One minute there are five metres of water underneath Investigator. In a few seconds, there are only five metres between its hull and the unforgiving ocean bottom.

Ludin and Joe Boucher, who steer the boat in shifts, have to gently throttle back to avoid shoals or rogue ice floes. If they slow the boat too quickly, the side-scan sonar device that it’s pulling on a heavy armoured cable will crash into the sea floor.

For 12 hours a day, Moore sits in a high arm chair on the Investigator, staring at a laptop computer as live sonar images, in varying shades of brown, slowly cascade down the screen.

On our day in Erebus Bay, the team works through rain, sleet, snow and warm sunshine, one hour up a search lane at six knots, another hour down the next search lane, rarely breaking speed.

They have nothing but cold sandwiches, tepid coffee and some REM, Van Morrison and Pink Floyd playing through a makeshift tablet speaker that Boucher fashions from a paper cup to break the monotony.

Any moment in this expedition can be the one the world has been waiting 166 years to reach: the first non-Inuit sighting of Erebus or Terror since Arctic whalers last saw them sailing off in search of the final leg of the Northwest Passage.

If the wood and iron wrecks are still relatively whole, ships that were just over 100 feet long when their epic voyage began in 1848 would each take up a few digital centimetres on Moore’s computer screen.

Turn away too long and he can miss history.

Investigator pulls a silver Klein System 3000, a small, torpedo-shaped sonar device, through the water. It’s called a tow fish and Moore has to fly it just above the undulating sea floor to get the best chance of finding shipwrecks without nosediving the sonar.

A black band up the middle of Moore’s computer screen marks the path of the side-scan sonar device.

If the black space begins to narrow, it means the bottom is coming up, and Moore quickly works a control knob with his left hand to winch in the tow fish before it hits something. When the hazard passes, he plays out the cable again to get a broader look.

There’s a startling variety of shapes to sift through on a sonar image of the bottom of Erebus Bay.

In several passes, we saw ice scour lines that formed circles, or networks of trenches that looked like freeways converging in Los Angeles, from an airline passenger’s window.

Others made a capital A. There were several huge, perfect Xs and some arrows, as if some undersea ghost might have marked the spot or pointed the way.

And there were ice scours like huge claw marks or lightning bolts.

“In 2011, we saw some ice scours that were 50 to 100 metres wide and about 30 metres deep,” Moore says.

But still no shipwrecks.

Seeing what ice does to the seabed, time and again over the centuries, you can’t help but wonder: what happened if Erebus and Terror ended up in the path of one or several ice blocks in what was likely more than a century and a half under water?

“Even if there are ice scours that postdate 1848, I don’t think they would have obliterated the ships,” Moore says.

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