THE oceans are playing tricks on us, it seems. In a bizarre twist of nature, the sea can suddenly, without warning, become lighter than air. Boats that normally float sink like a stone and are consigned to the seabed within minutes.
Methane gas is the culprit. Organic matter deep under the seabed generates methane which works its way up through the sediment over thousands of years. Pockets of gas can build up beneath the surface. Every once in a while, the pressure gets too much and the gas explodes.
鈥淲hen the gas bubbles up from the surface, it lowers the density of the water, and therefore its buoyancy,鈥� explains marine geologist Alan Judd from the University of Sunderland.鈥滱ny ship caught above would sink as if it were in a lift shaft.鈥� People jumping overboard in lifejackets sink too. No trace of boat or passengers remains on the surface.
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It鈥檚 not just a theory. Gas below the surface is a known hazard for oil rigs. If they hit a gas pocket while drilling, the resulting blowout can sink the rig. But until now no one has found tangible evidence of ships being sunk by escapes of natural gas.
Judd got interested in sonar surveys of Witch Ground, an area of sea about 150 kilometres north-east of Aberdeen, carried out in the 1970s. The area is riddled with pockmarks from escaping gas. Surveys show that one unusually large mark, Witch鈥檚 Hole, has something resembling a plume of gas bubbles in its centre.
He persuaded French oil company Total to carry out another survey of the area. When its probe collided with the 鈥済as bubbles鈥�, Judd realised the plume might instead be a wrecked ship. He was intrigued. Could the ship have fallen victim to methane?
After boarding the survey ship and leaving Aberdeen harbour on a cold, wet, miserable night, we woke up to a calm, clear morning in the middle of the North Sea. There was nothing in sight except a couple of oil rigs on the horizon. Straight away, the 6-tonne yellow sub was launched into the 鈥渕oon pool鈥�, a hole 5 metres wide in the rear of the ship.
In the operations room, we watched the sub鈥檚 progress on banks of screens as it dived 150 metres to the bottom. After a few minutes of fuzziness, the green-grey seabed came into view, riddled with the soft, circular depressions that are the signature of escaping gas.
The pilot used a joystick to carefully steer the sub in a large square around the area where we thought the wreck would be. After an agonising three hours, a series of shadows interrupted the regular pattern of the sonar scan, and the busy room fell quiet. 鈥淲e鈥檝e come to a much more disturbed seabed,鈥� Judd whispered to me. 鈥淭he scan shows we鈥檝e just come up and over something.鈥�
Cautiously, the pilot brought the sub back for a closer look. Suddenly the murky but unmistakable shape of a hull loomed out of the darkness.
The boat was still in one piece, upright on the seabed, but shrouded in fishing nets snagged from passing trawlers. Lumbering cod peered through the hatches, while ghostly white crabs feasted on fish trapped in the netting.
鈥淭he boat didn鈥檛 go in either end first, it went down flat,鈥� Judd notes. This meant that we could rule out a collision or hole in the hull, because then it would have sunk end first, like the Titanic. 鈥淚t looks as though the boat was swamped,鈥� agrees Prescott. 鈥淓ither methane bubbles or adverse weather could have resulted in it landing like this.鈥�
Judd scrutinised the seabed for signs of recent gas activity at Witch鈥檚 Hole. He was looking for two key signs鈥搈ats of sulphur-oxidising bacteria, and precipitates of calcium carbonate, which both depend on the presence of methane. He didn鈥檛 find them.
But the seabed inside the pockmark was extremely rugged, compared to the smooth surface elsewhere. And that could be due to recent gas releases. 鈥淚t is tempting to suggest that it is evidence of a catastrophic gas escape in the not too distant past.鈥�
Eventually we had to return to land, still uncertain why the boat sank. But we did confirm that there is a wreck sitting upright in the middle of Witch鈥檚 Hole, and methane is still a definite suspect. 鈥淔or a boat to have randomly landed within Witch鈥檚 Hole would be an amazing coincidence,鈥� Judd says. Although it is 100 metres across, it represents a tiny target in the whole of Witch Ground.鈥�
Now they have found the wreck, Judd and Prescott hope to identify it by searching through reports of missing trawlers from the first half of the 20th century. If the wreck can be identified and dated, weather reports and Mayday calls might give further clues to its demise. 鈥淲e鈥檝e just begun to scratch the surface of what evidence there is,鈥� Prescott says.
Granada TV plans to cover the investigation as part of a new 鈥淪avage Planet鈥� series which will be aired on British television early next year.