But wait a minute. How hot is it when it comes out of the exhaust? Haven't I read somewhere... ?

'Chris,' said van der Berg cautiously, 'after the water's gone through the reactor, does it all come out as steam?'

'What else could it do? Oh, if we run really hot, ten or fifteen per cent gets cracked to hydrogen and oxygen.'

Oxygen! Van der Berg felt a sudden chill, even though the shuttle was at comfortable room temperature. It was most unlikely that Floyd understood the implications of what he had just said; the knowledge was outside his normal sphere of expertise.

'Did you know, Chris, that to primitive organisms on Earth, and certainly to creatures living in an atmosphere like Europa's, oxygen is a deadly poison?'

'You're joking.'

'I'm not: it's even poisonous to us, at high pressure.'

'I did know that; we were taught it in our diving course.'

'Your – grandfather – was talking sense. It's as if we'd sprayed that city with mustard gas. Well, not quite as bad as that – it would disperse very quickly.'

'So now you believe me.'

'I never said I didn't.'

'You would have been crazy if you did!'

That broke the tension, and they had a good laugh together.

'You never told me what he was wearing.'

'An old-fashioned dressing gown, just as I remembered when I was a boy. Looked very comfortable.'

'Any other details?'

'Now you mention it, he looked much younger, and had more hair than when I saw him last. So I don't think he was – what can I say? – real. Something like a computer-generated image. Or a synthetic hologram.'

'The Monolith!'

'Yes – that's what I thought. You remember how Dave Bowman appeared to Grandfather on Discovery? Perhaps it's his turn now. But why? He didn't give me any warning – not even any particular message. Just wanted to say goodbye and wish me well...'

For a few embarrassing moments Floyd's face began to crumple; then he regained control, and smiled at van der Berg.

'I've done enough talking. Now it's your turn to explain just what a million-million-ton diamond is doing – on a world made mostly of ice and sulphur. It had better be good.'

'It is,' said Dr Rolf van der Berg.

53 – Pressure Cooker

'When I was studying at Flagstaff,' began van der Berg, 'I came across an old astronomy book that said: "The Solar System consists of the Sun, Jupiter – and assorted debris." Puts Earth in its place, doesn't it? And hardly fair to Saturn, Uranus and Neptune – the other three gas giants come to almost half as much as Jupiter.

'But I'd better start with Europa. As you know, it was flat ice before Lucifer started warming it up – greatest elevation only a couple of hundred metres – and it wasn't much different after the ice had melted and a lot of the water had migrated and frozen out on Farside. From 2015 – when our detailed observations began – until '38, there was only one high point on the whole moon – and we know what that was.'

'We certainly do. But even though I've seen it with my own eyes, I still can't picture the Monolith as a wall! I always visualize it as standing upright – or floating freely in space.'

'I think we've learned that it can do anything it wants to – anything we can imagine – and a lot more.

'Well, something happened to Europa in '37, between one observation and the next. Mount Zeus – all of ten kilometres high! – suddenly appeared.

'Volcanoes that big don't pop up in a couple of weeks; besides, Europa's nothing like as active as Io.'

'It's active enough for me,' Floyd grumbled. 'Did you feel that one?'

'Besides, if it had been a volcano, it would have spewed enormous amounts of gas into the atmosphere; there were some changes, but nothing like enough to account for that explanation. It was all a complete mystery, and because we were scared of getting too close – and were busy on our own projects – we didn't do much except spin fantastic theories. None of them, as it turned out, as fantastic as the truth.

'I first suspected it from some chance observations in '57, but didn't really take them seriously for a couple of years. Then the evidence became stronger; for anything less bizarre, it would have been completely convincing.

'But before I could believe that Mount Zeus was made of diamond, I had to find an explanation. To a good scientist – and I think I'm a good one – no fact is really respectable until there's a theory to account for it. The theory may turn out to be wrong – it usually is, in some details at least – but it must provide a working hypothesis.

'And as you pointed out, a million-million-ton diamond on a world of ice and sulphur takes a little explaining. Of course, now it's perfectly obvious and I feel a damn fool not to have seen the answer years ago. Might have saved a lot of trouble – and at least one life – if I had.'

He paused thoughtfully, then suddenly asked Floyd:

'Anyone mention Dr Paul Kreuger to you?'

'No. Why should they? I've heard of him, of course.

'I just wondered. A lot of strange things have been going on, and I doubt if we'll ever know all the answers.

'Anyway, it's no secret now, so it doesn't matter. Two years ago I sent a confidential message to Paul – oh, sorry, I should have mentioned – he's my uncle – with a summary of my findings. I asked if he could explain them – or refute them.

'Didn't take him long, with all the byte-bashing he's got at his fingertips. Unfortunately, he was careless, or someone was monitoring his network – I'm sure your friends, whoever they are, must have a good idea by now.

'In a couple of days, he dug up an eighty-year-old paper in the scientific journal Nature – yes, it was still printed on paper back then! – which explained everything. Well, almost everything.

'It was written by a man working in one of the big labs in the United States – of America, of course – the USSA didn't exist then. It was a place where they designed nuclear weapons, so they knew a few things about high temperatures and pressures.

'I don't know if Dr Ross – that was his name -had anything to do with bombs, but his background must have started him thinking about conditions deep down inside the giant planets. In his 1984 – sorry, 1981 – paper – it's less than a page long, by the way – he made some very interesting suggestions...

'He pointed out that there were gigantic quantities of carbon – in the form of methane, CH4 – in the gas giants. Up to seventeen per cent of the total mass! He calculated that at the pressures and temperatures in the cores – millions of atmospheres – the carbon would separate out, sink down towards the centres and – you've guessed it – crystallize. It was a lovely theory: I don't suppose he ever dreamed that there would be a hope of testing it.

'So that's part one of the story. In some ways, part two is even more interesting. What about some more of that coffee?'

'Here you are; and I think I've already guessed part two. Obviously something to do with the explosion of Jupiter.'

'Not explosion – implosion – Jupiter just collapsed on itself, then ignited. In some ways, it was like the detonation of a nuclear bomb, except that the new state was a stable one – in fact, a minisun.

'Now, very strange things happen during implosions; it's almost as if pieces can go through each other, and come out on the other side. Whatever the mechanism, a mountain-sized piece of the diamond core was shot into orbit.

'It must have made hundreds of revolutions – been perturbed by the gravitational fields of all the satellites – before it ended up on Europa. And conditions must have been exactly right – one body must have overtaken the other, so the impact velocity was only a couple of kilometres a second. If they'd met head-on – well, there might not be a Europa now, let alone Mount Zeus! And I sometimes have nightmares, thinking that it could very well have come down on us...