Quite when a wreck will leak oil depends largely on how fast its steel corrodes. "One millimetre a decade is the long-term average for corrosion," says MacLeod. "We've got a pretty good handle on this now." Marine engineers normally reckon that a ship would lose its structural integrity once its steel plate has lost between a quarter and a half its thickness. Most ships sunk during the second world war were made of plate between 19 and 25 millimetres thick.
However, the rate of corrosion depends hugely on the circumstances of individual wrecks. "It is vital to inspect and measure," says Rean Gilbert. "What may be true of one wreck is not necessarily the same for another." Most of the second world war wrecks were damaged when they were sunk, which is likely to have compromised their structural integrity. Any holes might also allow local currents to maintain a constant supply of oxygenated water, accelerating corrosion.
What's more, the use of different metals in ships can set up galvanic coupling, essentially creating a battery with the steel plate as the anode. This causes the steel plate to be eaten away, even if no other metals are in contact with it. "On one US wreck we found very high corrosion rates within 5 to 10 metres of the carpenters' store. When we investigated we found the store had 5000 copper nails in it," MacLeod says. On the other hand, accretions of calcium carbonate and organisms such as barnacles can form a protective coating on the steel that retards the rate of corrosion - though violent storms can strip off this coating.
A lot depends on how a wreck is lying on the seabed. If it is not sitting upright, the rate of corrosion can be a lot higher due to the stresses placed on its structure. "Ships are designed to float, not to be wrecked," MacLeod says.
'Peak leak' soon
The crucial question is just when a wreck has reached the point at which it will start to gush oil. "We don't have a lot of the answers," admits Lisa Symons, who heads a team at the US National Oceanic and Atmospheric Administration (NOAA) in Silver Spring, Maryland, responsible for potentially polluting wrecks. But the general picture is clearer. As steel plates corrode and lose their strength they reach the point where the next storm will trigger a catastrophic release of oil. MacLeod believes that in five to 10 years we will enter a period of "peak leak", which he expects to last for 50 years or so.
For most countries, these old wrecks are out of sight and out of mind, until they start to leak. In the early 1990s oil started to come ashore in NOAA's Gulf of the Farallones marine sanctuary, south of San Francisco. Initially blamed on pollution from passing ships, it continued to come ashore sporadically throughout the 1990s. In late 2001 there was another spill that went on for months. It killed about 50,000 seabirds and eight sea otters, and polluted 100,000 square kilometres of tidal marshes. Chemical tests showed that the oil was not from Alaska - the usual type carried by tankers passing the Californian coast - and that it matched previous mystery spills. So NOAA turned its attention to eight wrecks off the Californian coast. Divers and satellite imagery eventually pinpointed the source as the wreck of the Jacob Luckenbach.
Although not a wartime wreck, the Jacob Luckenbach is of a similar vintage, having sunk after a collision in 1953. Fifty years later, researchers found that the ship rocked from side to side on the seabed every time there was a heavy storm, releasing oil. NOAA and the US coastguards decided it would be best to remove the oil, so in 2003 salvors bored holes in the ship's tanks and heated the oil with steam lances to lower its viscosity, then pumped it out. This technology for removing oil, known as hot tapping, is well established but it can be expensive, depending on the state of the wreck. Emptying the Jacob Luckenbach cost $19 million.
It is important to NOAA to understand the potential impact of leaks on fisheries and marine sanctuaries, and their timescale, says Symons. Partly as a result of the Jacob Luckenbach incident, NOAA set up its Resources and Undersea Threats (RUST) database. This is a catalogue of the wrecks around the US coast that could cause oil spills, together with what is known about their oil load, location and state of corrosion. It lists some 1700 of these wrecks, their oil carried either as cargo or as bunker fuel. Taken together, they could potentially leak more than 15 times the amount of oil spilled by the Exxon Valdez. The wrecks are not evenly distributed around the coast: there are hotspots such as the Gulf of Mexico, where German U-boats were particularly active in the months after the US joined the war.
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