The Sea Empress ran aground at 2007 GMT on 15 February 1996
Ten years after a grounded tanker spilt 72,000 tonnes of oil in Britain's only national coastal park, experts say its ecology has only now recovered.
On 15 February 1996 the Sea Empress was holed below the waterline as it entered the Cleddau Estuary in west Wales.
Thousands of sea birds were killed and 120 miles of coastline contaminated.
A huge clean-up managed to reopen the beaches within months, but it has taken a lot longer for the fragile foreshore habitat to return to normal.
Marine biologist Robin Crump, a former director of Pembrokeshire's Orielton Field Centre, said while it was the emotive pictures of dead and oiled birds that captured the public's attention, it was the death of millions of limpets that threw nature out of sync.
In Sea Empress: 10 Years On, to be broadcast on BBC1 Wales on Wednesday to mark the tenth anniversary, he says: "It has recovered but it takes a surprisingly long time for the balance of nature to be redressed.
"Limpets are a key species , we call them the rabbits of the shore. They graze off the seaweed and off the surface of the rocks.
"Soon after the spill - within six weeks - the whole of the shore went green. Then after a year in came the brown seaweed because there were not enough limpets to keep the seaweed down.
"Just by killing one species, the limpet, the oil spill actually changed the whole ecology of the shore.
"Eventually, now 10 years on, we are almost back to the situation we were in before."
Dr Crump's assessment is supported by new research published on Wednesday by the Countryside Council for Wales.
Oil spill expert Jon Moore found numbers of seabirds had recovered to pre-spill levels.
While there were some localised rock pools that had not completely recovered, most shores and shallow seabed returned to normal in five years.
"There are now only a small number of places where the oil spill can be seen," said Mr Moore.
"These amount to small sections of shore where there are some tarry deposits and one or two places where it is possible that there is still some residue in the mud."
In the days immediately following the spill people were worried it would have a much greater impact than it did.
If the spill had been in May or June, the fall-out would have far worse
RSPB warden Ian Bullock said: "At the time nobody believed Pembrokeshire could recover."
Many studies were conducted on the effects, with one common theme that the damage could have been far worse.
Had the tanker run aground in May or June there would have been many millions more birds in the area.
Weather conditions were also favourable, taking the oil away from island reserves at Skomer and Skokholm and out into Carmarthen Bay and the Bristol Channel.
Mr Crump added: "The shoreline really has recovered remarkably well and there is almost a danger for people to think, well, oil spills can't be all that bad."
The Sea Empress: 10 Years On, BBC1 Wales, 15 February, 2245 GMT