'So the only thing we have to decide is: how can we protect ourselves, against something as powerful as the Monolith? Look what happened to Jupiter! And, apparently, Nova Scorpio...'

'I'm sure that brute force would be useless, though perhaps we should explore that option. Dr Kraussman – how long would it take to build a super-bomb?'

'Assuming that the designs still exist, so that no research is necessary – oh, perhaps two weeks. Thermonuclear weapons are rather simple, and use common materials – after all, they made them back in the Second Millennium! But if you wanted something sophisticated – say an antimatter bomb, or a mini-black-hole – well, that might take a few months.'

'Thank you: could you start looking into it? But as I've said, I don't believe it would work; surely something that can handle such powers must also be able to protect itself against them. So – any other suggestions?'

'Can we negotiate?' one councillor asked, not very hopefully.

'With what... or whom?' Kraussman answered. 'As we've discovered, the Monolith is essentially a pure mechanism, doing just what it's been programmed to do. Perhaps that program is flexible enough to allow of changes, but there's no way we can tell. And we certainly can't appeal to Head Office – that's half a thousand light-years away!'

Poole listened without interrupting; there was nothing he could contribute to the discussion, and indeed much of it was completely over his head. He began to feel an insidious sense of depression, would it have been better, he wondered, not to pass on this information? Then, if it was a false alarm, no one would be any the worse. And if it was not – well, humanity would still have peace of mind, before whatever inescapable doom awaited it.

He was still mulling over these gloomy thoughts when he was suddenly alerted by a familiar phrase.

A quiet little member of the Committee, with a name so long and difficult that Poole had never been able to remember, still less pronounce it, had abruptly dropped just two words into the discussion.

'Trojan Horse!'

There was one of those silences generally described as 'pregnant', then a chorus of 'Why didn't I think of that!' 'Of course!' 'Very good idea!' until the Chairperson, for the first time in the session, had to call for order.

'Thank you, Professor Thirugnanasampanthamoorthy,' said Dr Oconnor, without missing a beat. 'Would you like to be more specific?'

'Certainly. If the Monolith is indeed, as everyone seems to think, essentially a machine without consciousness – and hence with only limited self-monitoring ability – we may already have the weapons that can defeat it. Locked up in the Vault.'

'And a delivery system – Halman!'

'Precisely.'

'Just a minute, Dr T. We know nothing – absolutely nothing – about the Monolith's architecture. How can we be sure that anything our primitive species ever designed would be effective against it?'

'We can't – but remember this. However sophisticated it is, the Monolith has to obey exactly the same universal laws of logic that Aristotle and Boole formulated, centuries ago. That's why it may – no, should! – be vulnerable to the things locked up in the Vault. We have to assemble them in such a way that at least one of them will work. It's our only hope – unless anybody can suggest a better alternative.'

'Excuse me,' said Poole, finally losing patience. 'Will someone kindly tell me – what and where is this famous Vault you're talking about?'

History is full of nightmares, some natural, some manmade.

By the end of the twenty-first century, most of the natural ones – smallpox, the Black Death, AIDS, the hideous viruses lurking in the African jungle – had been eliminated, or at least brought under control, by the advance of medicine. However, it was never wise to underestimate the ingenuity of Mother Nature, and no one doubted that the future would still have unpleasant biological surprises in store for Mankind.

It seemed a sensible precaution, therefore, to keep a few specimens of all these horrors for scientific study – carefully guarded, of course, so that there was no possibility of them escaping and again wreaking havoc on the human race. But how could one be absolutely sure that there was no danger of this happening?

There had been – understandably – quite an outcry in the late twentieth century when it was proposed to keep the last known smallpox viruses at Disease Control Centres in the United States and Russia. However unlikely it might be, there was a finite possibility that they might be released by such accidents as earthquakes, equipment failures – or even deliberate sabotage by terrorist groups.

A solution that satisfied everyone (except a few 'Preserve the lunar wilderness!' extremists) was to ship them to the Moon, and to keep them in a laboratory at the end of a kilometre-long shaft drilled into the isolated mountain Pico, one of the most prominent features of the Mare Imbrium. And here, over the years, they were joined by some of the most outstanding examples of misplaced human ingenuity – indeed, insanity.

There were gases and mists that, even in microscopic doses, caused slow or instant death. Some had been created by religious cultists who, though mentally deranged, had managed to acquire considerable scientific knowledge. Many of them believed that the end of the world was at hand (when, of course, only their followers would be saved). In case God was absent-minded enough not to perform as scheduled, they wanted to make sure that they could rectify His unfortunate oversight.

The first assaults of these lethal cultists were made on such vulnerable targets as crowded subways, World Fairs, sports stadiums, pop concerts... tens of thousands were killed, and many more injured before the madness was brought under control in the early twenty-first century. As often happens, some good came out of evil, because it forced the world's law-enforcement agencies to co-operate as never before; even rogue states which had promoted political terrorism were unable to tolerate this random and wholly unpredictable variety.

The chemical and biological agents used in these attacks – as well as in earlier forms of warfare – joined the deadly collection in Pico. Their antidotes, when they existed, were also stored with them. It was hoped that none of this material would ever concern humanity again – but it was still available, under heavy guard, if it was needed in some desperate emergency.

The third category of items stored in the Pico vault, although they could be classified as plagues, had never killed or injured anyone – directly. They had not even existed before the late twentieth century, but in a few decades they had done billions of dollars' worth of damage, and often wrecked lives as effectively as any bodily illness could have done. They were the diseases which attacked Mankind's newest and most versatile servant, the computer.

Taking names from the medical dictionaries – viruses, prions, tapeworms – they were programs that often mimicked, with uncanny accuracy, the behaviour of their organic relatives. Some were harmless – little more than playful jokes, contrived to surprise or amuse Computer operators by unexpected messages and images on their visual displays. Others were far more malicious – deliberately designed agents of catastrophe.

In most cases their purpose was entirely mercenary; they were the weapons that sophisticated criminals used to blackmail the banks and commercial organizations that now depended utterly upon the efficient operation of their computer systems. On being warned that their data banks would be erased automatically at a certain time, unless they transferred a few megadollars to some anonymous offshore number, most victims decided not to risk possibly irreparable disaster. They paid up quietly, often – to avoid public or even private embarrassment – without notifying the police.