Heavy oil 'as viscous as chewing gum'

"The thing you have to remember is that oil isn't oil," said the man from British Petroleum. "It's not what you encounter in the shops. There are a lot of different types of it, in varying fractions."

The 60,000 tonnes of oil being shipped in the Prestige tanker that sank off the Spanish coast was heavy fuel oil, a dense, opaque petroleum derivative made from the unboiled material – the "bottoms" or residue – of crude oil that has been broken into smaller pieces.

Ian White, managing director of the International Tanker Owners' Pollution Federation, which monitors the effects of tanker spills, said: "Even at room temperature it's as viscous as chewing gum. Once you get down to the bottom of the ocean, and it sank in 3,500m of water, the water will be just a couple of degrees." That will make the oil even less liquid, he said.

"From experience, we know that it's very likely just to stay there. In 1997 a tanker broke in two off the Japanese coast and sank. That was also carrying fuel oil. The oil just sits in the tanks, and is very likely to do so for decades."

Fears that the tanks would rupture as the ship sank, spilling the oil into the ocean, are probably misplaced, Mr White said. "Pressure does increase as you get deeper, but it only really affects gases. All tanks have vents to let air out. As this sank quite slowly, we could expect that the air was replaced by water. That means there's no reason for the hull to rupture."

If the hull had been about to crumple, it would have done so on the way down. But the comparatively small volume of oil that has leaked out suggests that did not happen. The hull was also unlikely to rust and decay once it reached the bottom of the sea, Mr White said.