By Richard Black
BBC science correspondent
Scientists in North America have concluded that the environmental impact of the Exxon Valdez spill was more serious than previously believed.
Fifteen years on, the environmental impact of the oil spill is still serious
Their report, published in the journal Science, shows that many animals and plants have suffered long-term damage.
The spill, almost 15 years ago, deposited around 40,000 tonnes of crude oil into Prince William Sound, Alaska.
The Exxon Valdez spill, one of the largest of all time, is the most thoroughly researched spill.
The report combines findings by many scientists who've investigated different aspects of the 1989 disaster.
It finds that significant amounts of crude oil remain trapped in sediment on the sea bed, where it is still poisoning creatures such as mussels and clams.
When larger animals eat these creatures, they are also damaged; the population of sea otters has recovered to only half of its pre-spill level, and several duck species are also significantly reduced.
Concentrations of toxic chemicals from the oil remain high enough to damage fish eggs.
The researchers conclude that assumptions about oil damage have to be challenged.
Damage to birds and fish persists over long periods, they say, and oil does not automatically dissipate in the sea over time.
The conclusions of this study will be felt well beyond Alaska, and not least in Spain, where debates continue over the legacy of the Prestige spill just over a year ago, and the adequacy of the Spanish Government's response.
But the study has been questioned by ExxonMobil.
Vice President Frank Sprow said in a statement: "Hundreds of peer-reviewed studies conducted by researchers from major independent scientific laboratories and academic institutions resoundingly demonstrate the recovery of the Prince William Sound ecosystem and strongly contradict the news release's claim that wildlife and aquatic plants continue to suffer as a result of the 1989 Valdez oil spill."