The ocean is a rich blue. But suddenly we see what this crisis is all about

There may be few indicators to the scale of the US oil slick from land, but from the air it is a different story

The radio chatter, mostly from spotter planes, is all about "the source" – the point in the Gulf where the Deepwater Horizon rig once stood. Our pilot has to be more precise. We are headed towards 28.44 degrees north and 088.23 degrees west. Since leaving the shore, a low-hanging fog has developed, obscuring any view of the ocean.

We are 12 miles away from our destination when the cloud bank, like the cliffs of Dover, abruptly drops away once more to reveal the ocean. Two reporters and a cameraman who have been covering the BP spill for days and days without physically seeing any oil, let alone a slick, lean out of our helicopter.

So that's it. The ocean is a rich emulsion blue here – different from the muddy brown closer to the mouth of the Mississippi – but suddenly we are seeing what this crisis in the Gulf of Mexico is all about. What we see first are like blemishes on the sea surface, slivers of an oily sheen that might almost be burn scars on skin.

But there are shades of catastrophe out here, obvious from our breezy, chilly perch (the doors of the chopper have been taken off). The "scars", some with the rainbow colours of peacock feathers, slowly give way to something more shocking. Vast swathes of rust that at times turn almost Martian red.

The slick here looks like the deep cuts of the Grand Canyon, but painted on water. Striations of earth tones from vivid to dull. If the streaks of sheen pretend a certain beauty, this thicker, ruder stuff does not. It is vile; a terrible ruddy intrusion on a landscape that, but for us, should be virgin.

To witness the slick from 3,000ft up – our pilot Burt Lattimore, 46, is not about to violate the minimum altitude floor imposed by the US Coast Guard out here – is to understand at once what the scientists and politicians have been assuring us for the past week: this is very, very bad. It is not a mess that is going to go away; it has to go somewhere and, eventually, that means on to the shore.

Worse is the knowledge that the slick is still growing, being fed by those sheared-off pipes 5,000ft below the surface. The insult to the sea is not over, it is just getting worse.

The sky now clear, our Robinson R44 chopper, a teensy flying machine with barely enough room left for our camera lenses, brings us finally to the source. "Ground zero," my Austrian colleague blurts into the microphone at his lips. Indeed. Beneath us is a swirling, seaborne city of activity. As many as 60 different vessels are circling a large grey steel rig with a helicopter pad and a yellow crane.

It is not Deepwater Horizon, that lies tangled on the seabed below. Rather it is a new rig moved in by BP so that they can begin to drill a relief well that should eventually allow for the clotting of the leak from the first well. The drilling of the relief well is in its third day now.

A spotter plane zips by around half a mile to the south of us. Their job today is to see where the slick is going and to direct other aircraft to the parts where detergents can most usefully be sprayed. "Pick your poison," Burt says, noting that detergents aren't especially kind to the seas either. He says that on a previous trip out here last week, he saw four whales near the spill. One of them was on her side, clearly dying.

As Burt mentions our fast-emptying fuel tank – we make it home later by a matter of minutes – we circle the new rig several times. It is difficult to identify what each of the vessels below us are doing. Some are clearly working in pairs, connected one to the other by large booms that are scooping up the oil.

Others are preparing for the arrival here as early as today of the first of the giant steel coffers that BP will lower on to the worst of the leaks in the hope that the escaping oil can be captured and fed by pipe to tanker ships.

Up here you wonder what will happen if the boxes, more technically known as cofferdams, don't work. How much more of this stuff can the ocean absorb? While ruddy in some areas and shiny in others like polished stone, elsewhere the slick is almost invisible.

A large ship is moving at speed through an area of water, for example, that, to our untrained eye, seems uncontaminated. It is only after it has passed that the turbulence reveals that here, too, there is a layer of fouled water that gathers in streaks along the edge of the wake. The boats are like icebreakers, cutting channels though the mess.

We have seen how bad this is already. But as our pilot swings us north again, the evidence of the tragedy becomes even more obvious. The slick, in all its different hues and consistencies, just goes on and on. The scale is numbing. Probably we could fly for hours and not see the end.

We hit the cloud banks again, but then, as if someone higher than us wants to be sure we miss nothing, they part once more over the Breton Islands, a nature reserve that, according to the most recent reports, had not been directly hit by the oil. But today, now at just 300ft, we see that this is no longer the case.

The tiny isle is trembling with thousands of birds – gulls and pelicans mostly – and there is no mistaking what else we see. Oil, some in clumps the size of mop-heads, is now sliming its shores. The sand is blackening and the booms that have been laid here seem immune to the advance.

"Take us home, Burt." We have seen what we came to see and it is disgusting. For days we have been reporting on a calamity we had been unable to witness. Now we have. And it is lurid in its awfulness. My last sighting before we cross the edge of land again near New Orleans: a group of dolphins playing. How far away is the oil? Ten minutes flying, or less.