It was thanks to SOSUS that scientists had discovered it was anything but quiet in the ocean depths. In particular, the frequency range below sixteen hertz was deafeningly loud. To make the noise audible to humans, they had to play it at sixteen times its actual speed. Suddenly an underwater quake sounded like thunder, and humpbacks sang like twittering birds, while blue whales could be heard booming staccato messages to one another over hundreds of kilometres. Almost seventy-five per cent of the annual data recordings were dominated by a loud, rhythmic rumbling – the sound of airguns used by oil companies to explore the geological structure of the ocean floor.

Since then NOAA had added to SOSUS by developing its own systems. Every year the network of hydrophones probed further into the ocean. And every year the scientists heard a little more.

'We can ID an object just from its noise,' said Vanderbilt. 'That way we can find out how big a vessel is, how fast it's moving, what kind of power it uses, which direction it's coming from and how far away it is… Hydrophones tell us everything. It's a well-known fact that water conducts sound with great efficiency. Sound waves travel incredibly quickly under water – at speeds of up to five and a half thousand kilometres per hour. Which means that if a blue whale breaks wind off the coast of Hawaii, some guy in California will hear it in his headphones barely sixty minutes later. SOSUS doesn't merely detect sound; it identifies its source. NOAA's sound archive contains all the noises you could wish for: clicks, grumblings, whooshings, bubblings, squeaks, murmurs, bioacoustic and seismic data, and environmental noise. We can categorise pretty much everything – although there are a number of exceptions. And what do you know? Who have we got with us but NOAA's Dr Murray Shankar! He'll have the pleasure of telling you the rest.'

A thickset, timid man with Indian features and gold-framed glasses got up from the front row. Vanderbilt called up another spectrogram and played the artificially processed sound. The room was filled with a muffled drone, structured by gradual rises in pitch.

Shankar cleared his throat. 'We call this noise Upsweep,' he said, in a soft voice. 'It was first recorded in 1991, and its source seems to be located somewhere around 54 degrees south, 140 degrees west. Upsweep was one of the first unidentified noises picked up by SOSUS, and it was so loud that it was detected throughout the Pacific. We still don't know what it is. According to one theory, it may have been produced by resonance occurring between seawater and molten lava in an underwater range between Chile and New Zealand. The next images, please, Jack.'

Vanderbilt presented two new spectrograms.

Julia, recorded in 1999, and Scratch, detected two years earlier by an autonomous hydrophone array in the Equatorial Pacific. The amplitude was clearly audible within a radius of five kilometres. Julia sounds rather like an animal call, wouldn't you say? The frequency of the sound alters rapidly. It's broken down into a series of discrete notes, like whalesong.

But it can't be a whale. No whale could produce a noise of that volume. Scratch, on the other hand, sounds as though a needle is being dragged at right angles to the groove of a record, only it would take a record player the size of a city to create a noise like that.'

The next noise was a drawn-out creak that fell away slowly.

'Detected in 1997,' said Shankar. 'Slowdown. We think it originated somewhere in the South Pole. We've ruled out ships and subs. The noise might be caused by an ice plate scraping over the rocks in the Antarctic. Then again, it could be something else entirely. There's always the possibility that it's bioacoustic in origin. Some people are dying to see us use these noises to prove that giant squid really exist, but as far as I'm aware, creatures of that kind are barely capable of producing noise at all. It's a false lead, in my opinion. In any case, no one knows what Slowdown is, but. . .' He smiled shyly. 'Well, at least there's one rabbit we can pull out of the hat for you today.'

Vanderbilt played the spectrogram from the URA again. This time he turned on the sound.

'Did you hear what it is? It's Scratch. And can you guess what the URA told us? The noise was coming from the cloud! Well, from that we can-'

'Thank you, Murray, that was worthy of an Oscar.' Vanderbilt coughed and wiped his forehead with his handkerchief 'Those were the facts. The rest is speculation. OK, folks, let's round things off properly by giving you something to exercise your brains.'

THE SCREEN SHOWED images that had been taken in the darkest depths. Floating particles sparkled in the floodlights. Then a flat-looking creature billowed up into the frame, and instantly retreated.

'The film was cleaned up by Marintek, before the institute had the misfortune of being washed into the sea. Now, if you watch their version, two things are clear: first, this thing is enormous; and second, it glows, or, to be more precise, it flashes, then the light goes out when it enters the frame. All we know is that it was frolicking in the water seven hundred metres below sea level on the continental slope near Norway. Take a good look at it. Is it our jellified friend? Find some answers for us. The salvation of God's number-one species is in your hands.' Vanderbilt grinned at the rows of delegates. 'I'm not going to lie to you: we're heading straight for Armageddon. That's why I'm proposing that we share the work between us. You're going to put a stop to this mutated shit – find a way of taming it, feed it something that will make it spew its guts. Whatever. Meanwhile we'll find the bastard who's been sending us this crap. Do whatever it takes, but don't go shooting your mouth off. You can kiss goodbye to the thought of making headlines. A joint US-European policy of strategic disinformation is already in place. Panic would be the icing on the turd, if you know what I'm saying. The last thing we need is any social, political, religious or other unrest. So when we let you out to play, just remember what you promised Auntie Li.'

Johanson cleared his throat. I'd like to express my thanks on behalf of everyone here for a thoroughly engaging presentation,' he said pleasantly. 'Now let me get this straight. You want us to tell you what's lurking in the water.'

That's right, Doc.'

'And what do you suspect it is?'

Vanderbilt smiled. 'Jelly. And a few blue clouds.'

'I see.' Johanson grinned back. 'So you'd like us to open the advent calendar all by ourselves… You know what, Vanderbilt? I think you've got a theory. And if you want our co-operation, maybe you should tell us what it is. That's reasonable, isn't it?'

Vanderbilt rubbed the bridge of his nose. He exchanged a look with Li. 'Well, let's see now… Christmas wouldn't be Christmas without any presents,' he drawled. 'Aw, what the heck? The question we asked ourselves was this: where are the disaster hot spots? Which areas have got off lightly? Have any regions of the world been spared? And, hey presto, the unaffected areas are the Middle East, the former Soviet Union, India, Pakistan and Thailand. Plus China and Korea. You could count the iceboxes too, I guess, but I don't see much point. Basically, the main victim is the West. Take the destruction of the Norwegian offshore industry. That alone is going to do the West some serious long-term damage and put us in a very tricky position of dependency.'

'So, if I understand you correctly,' Johanson said slowly, 'you're suggesting it's terrorism.'

'Give that man a medal! You see, mass destruction is the hallmark of two types of terrorism. The first seeks to achieve political and social revolution, no matter what the cost, even if it means killing thousands in the process. Islamic extremists, for instance, think oxygen's too good for unbelievers. Type number two is fixated with Doomsday, and spreads the word that mankind is evil – we've outstayed our welcome on God's fair planet and deserve to be destroyed. The more money and technology these people can get their hands on, the more dangerous they become. Take killer algae, for example. Someone out there must be capable of breeding that stuff. Everyone knows how to train a dog to bite. Gene technology lets us tamper with DNA, why can't we use it to modify behaviour? Think about it… So many mutations in so little time. How does that look to you? If you ask me, someone's been very busy with their test-tubes. We've got an unknown shapeless organism out there too. Why is it shapeless? Everything's got a shape! Maybe it doesn't need one for its purposes. Maybe it's a kind of protoplasm, an organic compound, a sticky mess that channels itself in tiny strands like molecular chains, setting up home in whale brains and lobster shells. You know, folks, this definitely isn't coincidence. This is design. And if you want a motive, just think what the collapse of the European oil industry will do for the Middle East.'