The world's most damaging oil spill - now in its 41st continuously gushing day - is creating huge unseen "dead zones" in the Gulf of Mexico, according to oceanologists and toxicologists. They say that if their fears are correct, then the sea's entire food chain could suffer years of devastation, with almost no marine life in the region escaping its effects.
While the sight of tar balls and oil-covered birds on Louisiana's shoreline has been the most visible sign of the spill's environmental destruction, many scientists now believe it is underwater contamination that will have the deadliest impact. At least two enormous submerged clouds of noxious oil and chemical dispersants have been confirmed already by research vessels, and scientists are seeing the initial signs of several more. The largest of these is some 22 miles long, six miles wide and 3,300 feet deep - a volume which would take up half of Lake Erie. Another spans an area of 20 square miles.
More than 8,300 species of plants and animals are at risk of harm. Some, such as the bluefin tuna which come to the Gulf to spawn at this time of year, could even face extinction. Scientists predict it will be many months - or even years - before the true toll of the disaster will be known.
In previous spills, oil rose to the surface and was dealt with there, but due to the use of dispersants - as well as the weight of this particular crude oil and the pressure created by the depth of the leak - much of the oil has stayed submerged in clouds of tiny particles. At least 800,000 gallons of dispersants were sprayed at escaping oil in a frantic attempt to keep it offshore, but it now seems this preventative measure has created a worse disaster. The chemicals helped to keep the oil submerged and are toxic to marine life, resulting in unprecedented underwater damage to organisms in the Gulf.
Once these harmful substances enter the food chain, almost nothing will escape their effects. Forests of coral, sharks, dolphins, sea turtles, game fish and thousands of shellfish could all face destruction. What happens next to these underwater clouds - or plumes as some scientists have called them - depends largely on the currents they are caught in. If they do eventually rise to the surface, they may end up on the shoreline months or years from now, causing a second wave of destruction to delicate wetlands.
The leak itself is far from over. With up to 40 million gallons of oil swilling in the sea, efforts to plug the hole (disgorging up to 19,000 barrels a day) have become frantic. Since Wednesday, BP has been trying to block the source by blasting it with mud and concrete. On Friday things took a more desperate turn as BP added a scientifically dubious-sounding "junk shot" of shredded rubber and golf balls. Whether any of this has worked will probably not be clear for some days, but as BP continues to delay an announcement, everyone is getting jittery.
"It's the biggest environmental disaster of our time and it's not even over yet," said the marine toxicologist Dr Susan Shaw, director of the Marine Environmental Research Institute based in Maine. She has been diving among the damage and is horrified by the contamination caused by BP's continued use of dispersants. "The dispersants have been used at such a high volume that it's unprecedented. The worst of these - Corexit 9527 - is the one they have been using most. That ruptures red blood cells and causes the fish to bleed. With 800,000 gallons of this, we can only imagine the death that will be caused."
According to Dr Shaw, plankton and smaller shrimps which are coated in these toxic chemicals will be eaten by larger fish, passing the deadly mix up the food chain. "This is dismantling the food web piece by piece," she said. "We'll see dead bodies soon. Sharks, dolphins, sea turtles, whales: the impact on predators will be seen in a short time because the food web will be impacted from the bottom up."
The largest of the clouds, confirmed by a University of South Florida research ship last week, has gone deeper than the spill itself, defying BP's assurances that all oil would rise to the surface. It is now headed north-east of the rig, towards the DeSoto Canyon. This underwater trench could channel the noxious soup along the Florida coast, impacting fisheries and coating 100-year-old coral forests. Tests on the toxicity of another chemical cloud, some 10 miles long and heading south-west of the site, are also being done by scientists from the University of Georgia.
Marine biologists say the timing of this underwater contamination could not be more catastrophic. "This is the time of year when all the animals are reproducing and hatching, so the damage at this depth will be much worse," said Dr Larry McKinney, director of the Harte Research Institute for Gulf of Mexico Studies in Texas. "We're not talking about adults on the surface; it will impact on the young - and potentially a generational life cycle."
This interrupted life cycle could wipe out more precarious species in the region. "Bluefin tuna spawn just south of the oil spill and they spawn only in the Gulf. If they were to go through the area at a critical time, that's one instance where a plume could destroy a whole species. What's happening in Louisiana on the wetlands is very bad, and we can see that, but sometimes it's the things you don't see that can kill. These underwater plumes may be the more serious threat for the entire Gulf."
What happens next to these suspended clouds is what worries scientists. Nobody knows how long it will take for them to reach the surface and come towards the shore (if they ever do), and they might wash up on land and undo all the work of the initial clean-up.
Dr Peter Roopnarine, an invertebrate zoologist and geologist at the California Academy of Sciences, is conducting tests on molluscs to gauge oil damage. He fears a second wave of wetland damage from these sub-surface plumes. "The organisms we're working with are in shallow sub-tidal waters and into the salt marshes, so we won't get immediate results from a plume. But we could end up seeing two disasters on shore, because the plume will eventually work its way to the surface and to the shore."
With no confirmation that BP's attempts to stop the flow of oil have succeeded, the damage is likely to get worse. If this "top kill" method of plugging the hole with concrete and mud fails, then the only option left is a relief well, which will take until August at the earliest to become operational. In the meantime, the surrounding ocean will become deadlier with every day that passes. And even if the plug works, it may well be too late. As Dr McKinney pointed out, "This has every possibility of being the worst oil spill of all time. At the depth that these plumes are at, the sea will be toxic for God knows how long."