An unnamed diver accompanied Dr. Susan Shaw, director of the Marine Environmental Research Institute and an expert in the toxicology of marine mammals, in a onsite dive at the Gulf Disaster, and wrote a long unsigned report for the Times of London posted on Friday.

Thirty feet down in the Gulf of Mexico, the water suddenly goes dark. Above me, a ragged black silhouette of oil is eclipsing the sunlit circle that is the surface. I feel a familiar twinge in the back of my brain, the first warning that soon I need to get back up to breathe. With concentrated calmness, I pause and hang in the water, staring. Oil is above, around and beneath me; out of sight somewhere way below lurk yet more dark clouds of oil. This is now America’s worst environmental disaster, according to the White House. The beaches are clogged not only with oil, but with clean-up workers, TV crews and politicians. But it is here, beneath the slick, that its destructive power will be most felt.

The silted tongues of the Mississippi river don’t reach this far from shore, leaving the ocean clear. The familiar, cavernous blue of deep water stretches below, occasionally probed by a wavering beam of sunlight that has pierced the armour of the slick above. For as far down as I can see, there are tiny specks glinting in the shifting light, while others lurk as shadows behind.

Yellow-brown haloes surround the light when it appears from above, its fringes dotted as though by a swarm of motionless insects. Ten feet below this haze, I force my eyes to focus close to my mask. A shred of the slick hangs in the water in front of me, the size of a snowflake, but one that is a lurid, toxic brown. Inches beyond it are more of them, melted into a variety of ghoulish shapes. The chemical dispersants being sprayed from the air are dragging the oil away from the surface in scraps, and now they are arrayed in sinister formation, the largest hovering just beneath the underside of the slick, the smallest way below. In British waters, a sight like this would mean plankton, evidence of a huge bloom of new life. Here in the Gulf, it looks like death.

Looking up, my stomach lurches. I can no longer see the waves above. If I come straight up now, I’ll be coated in the poisonous sludge.

But what has spooked Shaw more than the oil is the spectre of what has been sprayed on the oil to keep it from reaching the valuable – and highly visible – coastlines. The dispersants are designed to break up the oil and lessen the physical damage. Unfortunately, they have an environmental cost of their own. BP initially responded to the growing spill by spraying Corexit 9527 containing 2-butoxyethanol, a chemical that ruptures red blood cells when ingested. “When it was sprayed over the Exxon Valdez spill in 1989, workers suffered health problems, including blood in their urine, and liver and kidney disorders,” says Shaw.

She goes on to explain that its replacement, Corexit 9500, contains compounds that are acutely toxic to invertebrates, and can cause chemical pneumonia if aspirated into the lungs following ingestion. By the time this magazine went to press, more than one million gallons of dispersant had been sprayed on to the slicks. What we’d seen beneath the slick hanging in the water was a toxic cocktail of light sweet crude and dispersant, masquerading as plankton. As soon as it was ingested by grazing fish, it would start a journey from the body of prey to that of predator, and on up through the food chain, getting more concentrated at every step.