The writhing ball of sardines sways to and fro just below the surface, like a single restless organism. Suddenly one streamlined shape, then another, cuts across the frantic shoal. The new arrivals have barbecue-grill markings and snouts elongated into lethal weapons. The Blue Planet began on BBC1 on 12 September. See The Blue Planet Challenge on the Nature page of www.bbc.co.uk. There will be presentations on the making of the series at Dive 2001.
They move fast - capable of up to 70mph, few animals could match their speed. Soon several dozen have gathered for breakfast, twisting and slashing at the baitball with their spear-like probosces.
The striped marlin are followed to the feast by smaller pelagics - juvenile yellow-fin tuna, the distinctive gold bands along their sides flashing in the sunlight. The cameramen hang, one at the surface, the other breath-holding, supplemented by sips from a 4 litre pony bottle.
They can't believe their good fortune, but then the cream on the cake arrives in the shape of a stately 14m sei whale. It heads steadily for the baitball, and opens its mouth wide. By the time the diners have dispersed, all that's left are sardine scales falling like snow...
The only humans who normally witness such underwater sights are the few free-diving maniacs prepared to hang around for hours in mid-ocean, spearguns ready. But the film-makers have waited far longer for their own magic moment - around four years.
There is a loud buzzing noise behind me. Alastair Fothergill reaches forward suddenly and closes his fist on something airborne in the vicinity of my right ear.
He spreads his fingers to reveal a surprisingly large insect with iridescent green wings. 'Is that a beetle?' murmurs Alastair. 'Exquisite.'
We're at BBC Bristol, where the wonders of the world, from beetles to baboons to blue whales, exist to be dished up on our TV screens.
No unusual creature can hide from the probing lenses of the Natural History Unit, so they have obviously taken to turning themselves in voluntarily.
We're not on the roof garden to discuss beetles, however, but Fothergill's current project, a documentary series no diver dare miss.
Alastair learned to dive for the Reefwatch series fronted by fellow-producer and member of the NHU Diving Team Martha Holmes, back in 1988. He later won acclaim for his polar wildlife documentary series Life in the Freezer.
People liked the fact that those programmes took them somewhere they would probably never visit, where man was dwarfed by nature. 'Afterwards I looked around for another habitat of that sort, and I believe the oceans are the final frontier,' says Fothergill. 'No-one had ever made a major series that explained the whole ocean system. In a sense that's crazy, because the oceans are completely united. I also like making elemental films, where the power of nature can be seen on the screen.'
The result, The Blue Planet, has taken five years to make and is the most ambitious wildlife documentary in TV history. Had the BBC taken much persuading to do it? 'No, it was very easy, but I was adamant that I wouldn't compromise on the budget. There was no point doing what had been done before.'
That£7 million budget might represent a big chunk of licence-payer's money, but the series has already been sold in 40 countries and will become course material for marine biology students. Then there's the cinema version, the kids' film, the soundtrack CD, the DVD, book, children's book, website, even a gala concert at the Festival Hall...
No expense has been spared, from the 7000 hours of videotape which took a year to edit to George Fenton's Hollywood-blockbuster score.
'This is a big, big film!' says Fothergill. 'Divers will be blown away by it.'
And he's right - the underwater footage is gobsmackingly good. I fear it will deal a death-blow to all those unedited amateur videos of somebody's mate posing by a rock with some wrasse.
The eight 50-minute programmes, narrated by David Attenborough, cover all aspects of the ocean world. Divers can enjoy a fresh perspective on familiar sights in episodes such as Coral Seas, Tidal Seas and Seasonal Seas and Coasts, and will be fascinated by experiences filmed in those waters few of us will ever get to visit in person.
In Frozen Seas, for example, Martha Holmes takes us into the polar waters of both the Arctic and Antarctic. In The Deep, we see unique footage of the volcanic vents from which some believe life on Earth sprang. And in Andy Byatt's Open Ocean, we witness the sort of spectacular action that divers crave.
To explain what went into making the programmes, a nine-minute Making Waves segment concludes each episode. Finally, a 50-minute special called Deep Trouble examines the threat posed to the oceans by fishing.
The Blue Planet might have broken records for time and money spent, but it also used a relatively small team. 'On a classic Attenborough series we might put 50 cameramen into the field, but there are far fewer really good underwater cameramen,' says Fothergill.
He used 15, and says half the series was shot by only three - Peter Scoones (mainly coral reefs), Doug Allan (a polar specialist) and the American Rick Rosenthal, whose rapid-response free-diving skills came in handy when filming in the open ocean.
The teams visited more than 200 locations. Of Martha Holmes' Coral Seas, for example, Fothergill says: 'This is the ultimate coral-reef film - we have done a sequence in literally every coral-reef system around the tropics.'
Why not save money and stick to the Red Sea? I don't have time to put the question - Fothergill, clearly fore-armed, raises it himself: 'People have done that, but if you're going to try to film, say, the best examples of coral or fish spawning, competition for space or whatever, you would choose many places round the world to get it.
'Coral reefs are familiar and Martha really wanted to push the boundaries.' So, for example, innovative sequences include time-lapse footage of corals eating one another, phosphorescent corals, and whitetip reef sharks hunting by night.
Much of the macro filming throughout the series was shot on location rather than using aquarium set-ups, so that small creatures would not be disturbed and would behave more naturally.
At the other end of the scale, ever wondered why those nice orcas are called killer whales? In the introductory programme, Fothergill's team has captured images he thinks will be the defining sequence of the series, in which a pod of killer whales run down a grey whale mother and kill her calf after a six-hour hunt - just for the hell of it. 'That was a very emotional experience,' he says.
'I'm proud that there's so much stuff you've never seen before. That's relatively rare nowadays, because we've already filmed a lot of the natural world.
'Divers appreciate it, but other people don't always understand the massive technical problems involved in filming something like the vents a mile or more under the sea.
'That programme, The Deep, is crammed with new stuff. Over 60 per cent of the animals have never been filmed before and a lot are new to science.'
Only half a dozen scientific submersibles can dive to such depths, and hiring them is costly. Alastair Fothergill solved the problem by offering the boffins the chance to let the professionals sort out the filming and reveal their subjects in a new light.
'The scientists loved the idea,' says Fothergill. 'Most of the subs had pretty poor Hi-8 camera systems, so we invested initially in Digi-Beta and later high-definition cameras in special housings. Then we put cameramen on board who could frame up, get the close-ups and spend time improving the quality of the images.
'We put lights out on longer arms to get cross- and back-lighting and made it look much nicer. There's wonderful macro photography of tubeworms on the cold vents which has never been professionally photographed before, but it looks as if it's been filmed in a tank in Bristol.
'The stuff we got from the hot vents has been very useful to scientists, because they've never seen it shot so close up and so beautifully.'
Confronted by such results, I was puzzled by rumours I had heard that the Blue Planet team had failed to get the footage it was after. Alastair seems taken aback. 'We knew early on that a lot of things were going wrong, which is probably the gossip you picked up,' he says. The planned four-year project had turned into five, though he blames that on a transmission scheduling decision.
'We were shooting on video for clarity, but video cameras break down all the time, and there were a lot of disappointments where we got everything else right but the gear would let us down.
'Divers are very familiar with this. In the oceans the animals have to behave, the light's got to be good, it's got to be calm enough to dive, the gear's got to behave - there are so many things to go wrong. I think we were ground down by it. At any one time I was running an office that might have five or six difficult major trips in the field.'
But he believes the rumours arose mainly from difficulties experienced in making the Open Oceans programme.
'Last march, when we should have finished all the filming, Andy Byatt and I sat down here and reckoned we had only about 20 minutes of a 50-minute film. Not because Andy isn't a good producer, but because we had had failed trip after failed trip.
'We're used to failure in this business, but if you go to a waterhole in Africa, in the end a lion will come to drink. With the open ocean, if you don't find your whale, your marlins, your baitball, it's a desert - there's nothing there.'
Andy Byatt is an ebullient character, a geologist by training and a member of the NHU diving team. He started diving in 1983 in Scotland, became a BSAC Instructor and later a commercial diver, and got into TV as a support diver for Martha Holmes. His brief to produce the Open Oceans programme for The Blue Planet must count as one of the great nerve-testers of all time.
'For two years we shot virtually nothing,' he tells me. 'We were trying to get any big pelagic fish, but we had about five minutes' worth of film. I didn't have many fingernails left.
'By the time we'd been through the entire three-year shooting schedule, we still had only glimpses of white and blue marlin and a smidgen of footage.'
Transport was always a problem. Dive-boats big and fast enough to take the crew into open ocean cost£2000 a day, and as Fothergill says, 'when you're getting nothing, that's frightening. We were caught halfway: we couldn't afford the Aggressors, but we needed a bigger boat than those at the bottom end of the market. Only a few can do a really decent service for six divers.'
Once alastair had juggled his funds and secured another boat, there would be two ways of making the wide open spaces yield their secrets. The naturalistic, preferred way, meant following plankton and watching for seabird activity or water-quality changes. 'You can go for weeks and see nothing that way,' says Andy Byatt.
The other way was to lure fish to the boat, as sports fishermen would. 'It's artificial behaviour but you can get some fantastic footage. It's unbelievably exciting but no one should attempt it without real knowledge of the fish and full control of the boat!'
An overdose of action was hardly Byatt's problem, however. 'I never got really downhearted,' he says, but after yet another fruitless effort in the Azores he had shouted at the skies: 'If there's anybody up there, help me out!' The following day a gale set in for the remaining fortnight of the trip.
Last summer, when filming should have been long over, he started capturing some stunning sequences - dolphins joined by shearwater birds diving to 15m to participate in a mackerel massacre; a sailfish making life hell for a huge ball of anchoveta; rays and sharks going about their business around sea-mounts; a pilot whale and oddities like half-moonfish and floating crabs.
Then, in November, a three-week trip off Mexico saw his prayers finally answered. 'Suddenly we got the lot. We dived and filmed every day but one, when the wind blew up.
'That was Thanksgiving Day, which was perfect for our American crew and cameramen.' They had plenty to be thankful about.
Finally, the weary crew asked Andy Byatt what more he could possibly ask for. 'I said: 'What I really want is gin-clear water, 20 or 30 marlin working a baitball, then I want the tuna to come in - if there's a dolphin or shark so much the better - and at the end, of course, a sei whale.'
'And that was exactly what happened on the last day. We even got sea-lions and brief encounters with dusky sharks and common dolphins, plus the ultimate sei whale experience. From the deck, the ocean was erupting. I've had blessed trips, but never anything like that.'
Andy Byatt reckons there's a wider lesson for divers in Open Oceans. 'I could take that experience and do it off the coast of Britain and I would see some fantastic things,' he says. Given unlimited time, of course.
'Divers are always looking for glamorous locations, which basically means coral diving, lots of bright colours - easy,' he goes on. 'I can't stress enough what's outside your back door. You don't even need a boat, there's great shore-diving to be had. Some of the best diving I've had was in the Farne Islands and the west coast of Scotland.'
Just hanging out? 'Perhaps in the Med, just snorkelling offshore as the sunlight starts to get to an acute angle, there's a real possibility of being with dolphins, or just enjoying the plankton - when it's backlit it's absolutely beautiful.'
That there were no serious diving incidents during the making of The Blue Planet is a tribute to the unit's exacting safety controls, based on HSE standards but taken several steps further. In fact the hairiest moments came at the surface, when Doug Allan and Sue Flood were filming on an ice floe that broke loose and floated out to sea. Fortunately they were able to maintain radio contact, and were rescued by helicopter after a nail-biting 12 hours.
'As divers we're quite conservative and just had the odd run-ins with beasts,' says Andy Byatt. 'Some situations can be quite tense. Bluewater diving is very agoraphobic, for instance, and some people can react badly on a psychological level. We did a lot of work with sharks, too, and whether it's safe or not again comes down to psychology.
'When you're paddling round next to major predators, you're relying very much on the fact that you're not their normal meal.'
Marlin or sailfish, too, he says, could intimidate if you let them. 'It's OK when you're behind the camera, but when you come out and see that sword inches from your head, you think I really wouldn't fancy being impaled on that!' And get too involved in a giant baitball and it can swallow you and make you its nucleus, after which anything might happen.
Many open-ocean and other sequences were shot using minimal diving equipment, to allow divers to get in and out of the water fast and not scare the animals.
'The fish might tolerate you for a while if you started bubbling but the intensity of activity would drop right off and the big fish would drift down. So one cameraman would be on a pony bottle, the other one snorkelling, and it's no coincidence that that sei whale was as close to them as I am to you.'
Rebreathers were often used on The Blue Planet when it was important to be non-intrusive - 'bubbles do piss fish off and they behave quite differently'. They also provided the chance of bubble-free low-angle images.
Draeger and US Bio-Marine were the rebreathers of choice, the latter because, although fully closed-circuit, it left the diver two hands free to use the cameras. 'Having a device where you have to manually inject the diluent gas is asking for trouble,' reckons Byatt.
But for much of the time 'plain standard scuba with real air as opposed to nitrox was the best tool, because it's simple and a lot of the stuff we were shooting didn't give a toss about bubbles,' he says.
There were also safety issues when using rebreathers in some of the more remote locations, 'though surprisingly we weren't often more than 12 hours from a chamber.'
A mere half-day? 'The only question then is how good is the chamber and the person running it.'
At the start of the project, the team had expected technological advances in diving equipment to make all the difference to its success. In the event, however, that was hardly an issue.
'The key thing was always the in-water ability of the dive teams,' says Andy Byatt. 'Knowing where to find the animals, getting into their mindset and being able to interact with them in a low-key fashion - that was what really mattered.'
It might have taken a while to get the results, but the success of the divers is now there for all to see.