So is it now safe to go back into the Gulf of Mexico? A year after the Deepwater Horizon blow-out – which killed 11 workers and spilled 4.9 million barrels of oil – it might seem so.
Beaches along the coast look like they're back to their breathtaking normal, the ecologically sensitive Louisiana wetlands seem full of life again, and resorts are hoping that tourists will start flocking back. Only 0.4 per cent of American waters there are still closed to fishing – down from more than a third last summer – and prawn catches were actually nearly 10 per cent higher in January and February than at the same time last year.
Two weeks ago, the Obama administration gave the first go-ahead for a new deep water well since the disaster (it had already issued permits for six previously approved ones). The clean-up force has been cut from 52,000 to 6,000, and, two months ago, the head of the government's special claims fund said research he had commissioned showed that the area would have almost fully recovered by next year.
And yet we are still only near the beginning of the story – for oil spills, like other environmental emergencies, have a short acute phase, followed by a long chronic one. As so far at Fukushima, the acute phase has gone better than once seemed possible – but the long-term consequences remain unknown.
Mercifully, even miraculously, the Gulf has been spared the devastation that looked all too probable in the early weeks of the crisis, when the gushing oil seemed unstoppable, and the winds were blowing an ever-growing slick straight towards the wildlife-rich marshes of the Mississippi Delta. The winds changed just in time and – together with favourable currents and the flow of the great river itself – held the oil offshore long enough for it to dissipate: BP's spraying of 1.84 million gallons of dispersants also helped.
But while catastrophe was averted, the task of assessing the true toll is only now starting. It is highly charged, both commercially – since the result may decide how much may have to be paid in compensation – and politically, since Barack Obama, damaged by his hesitant handling of the crisis, has been over-eager in declaring it over.
It is worth bearing in mind that the effects of the acute stage are more serious than they might appear. One hundred dead cetaceans, for example, washed ashore – but, as a rule of thumb, 50 times as many such whales and dolphins sink at sea, making the likely toll around 5,000. Similarly the 8,065 oiled birds recovered are bound to be only a small fraction of those affected; in a ghoulish exercise, researchers will dump avian carcasses overboard this summer to see what proportion make it to land through the shark-infested sea.
Nor are all the beaches as idyllic as they appear to be. Many have an oil layer beneath the sand, while others are strewn with tiny fragments of tar balls. Huge mats of weathered oil are plaguing surf zones where the waves crash in. Parts of the wetlands are seriously contaminated, too.
There may be other surprises in store. For several years after the 1989 Exxon Valdez spill in Alaska's Prince William Sound, the herring population seemed to have survived – but then crashed, never to recover. Birds that fed on affected shellfish in the area have had trouble breeding. And follow-up studies after a 1969 spill off Massachusetts found crabs still badly affected four decades later.
The biggest – and most hotly contested – issue is particular to this accident, which uniquely took place nearly a mile beneath the sea. Scientists increasingly expect that the greatest effects will take place in the deep ocean, but determining them, in the words of one US government expert will be "probably one of the most challenging things ever".
The official American position is that "most of the oil is gone" and, indeed, Department of Energy research suggested that naturally-occuring microbes did a good job of gobbling it up. But Prof Samantha Joye, of the University of Georgia – who has actually been to the sea floor in a submarine many times before and after the accident – tells a different story after finding an enormous "graveyard" covered in a thick coat of pollution. She reckons that the microbes managed to munch up only a tenth of the oil.
Perhaps most ominously is anecdotal evidence of illnesses among clean-up workers and other Gulf Coast residents, with blood containing elevated levels of the chemicals found in oil. A $19 million official study of 55,000 people has been launched to determine any health effects.
True, it could all have been so much worse. But, a year on, the story of Deepwater Horizon is still far from coming to a close.