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.
It sounds like a fisherman’s worst nightmare. But it could be true.
Scientists have just sailed out into the North Sea and found evidence that this
may have been the fate of a least one unlucky trawler. And I was there to
witness their detective work at first hand.
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.
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“When the gas bubbles up from the surface, it lowers the density of the
water, and therefore its buoyancy,” explains marine geologist Alan Judd from the
University of Sunderland.”Any ship caught above would sink as if it were in a
lift shaft.” People jumping overboard in lifejackets sink too. No trace of boat
or passengers remains on the surface.
It’s 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’s 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 “gas bubbles”, Judd realised the plume
might instead be a wrecked ship. He was intrigued. Could the ship have fallen
victim to methane?
To find out, he teamed up with maritime historians Robert Prescott and Mark
Lawrence from St Andrew’s University, marine survey company Fugro UDI and
British television company Granada TV to investigate the wreck. The team sailed
out to the site last week and invited me along for the ride. Their plan was
simple: to send down a remote-controlled, uncrewed sub equipped with sonar and
video cameras to find the wreck, and try to work out why it sank.
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 “moon pool”, a hole
5 metres wide in the rear of the ship.
In the operations room, we watched the sub’s 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. “We’ve come to a much more disturbed seabed,” Judd
whispered to me. “The scan shows we’ve just come up and over something.”
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.
Prescott began to describe the boat. “It’s a steel-built vessel, about 25
metres long . . . there’s not much damage . . . built between 1890 and 1930 . .
. there are the ship’s sides, and the deck.” But how did it get there?
“The boat didn’t go in either end first, it went down flat,” 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. “It looks as though the boat was
swamped,” agrees Prescott. “Either methane bubbles or adverse weather could have
resulted in it landing like this.”
Judd scrutinised the seabed for signs of recent gas activity at Witch’s Hole.
He was looking for two key signs—mats of sulphur-oxidising bacteria, and
precipitates of calcium carbonate, which both depend on the presence of methane.
He didn’t 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. “It is
tempting to suggest that it is evidence of a catastrophic gas escape in the not
too distant past.”
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’s
Hole, and methane is still a definite suspect. “For a boat to have randomly
landed within Witch’s Hole would be an amazing coincidence,” Judd says. Although
it is 100 metres across, it represents a tiny target in the whole of Witch
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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. “We’ve just begun to scratch the
surface of what evidence there is,” Prescott says.
Granada TV plans to cover the investigation as part of a new “Savage Planet”
series which will be aired on British television early next year.
