Van der Berg started to laugh, then found it wasn't easy inside his mask.

'So you still don't know what this is about?'

'I'm beginning to feel I'm the only one who doesn't.'

Van der Berg took his companion by the shoulder, and turned him to face the looming mass of Mount Zeus. From this distance, it filled half the sky – not merely the greatest, but the only mountain on this whole world.

'Admire the view just for one minute. I have an important call to make.'

He punched a code sequence on his comset, waited for the READY light to flash, and said: 'Ganymede Central 109 – this is Van. Do you receive?'

After no more than the minimum timelag, an obviously electronic voice answered:

'Hello, Van. This is Ganymede Central 109. Ready to receive.'

Van der Berg paused, savouring the moment he would remember for the rest of his life.

'Contact Earth Ident Uncle 737. Relay following message. LUCY IS HERE. LUCY IS HERE. End message. Please repeat.'

Perhaps I should have stopped him saying that, whatever it means, thought Floyd, as Ganymede repeated the message. But it's too late now. It will reach Earth within the hour.

'Sorry about that, Chris,' grinned van der Berg. 'I wanted to establish priority – amongst other things.'

'Unless you start talking soon, I'll begin carving you up with one of these patent glass knives.'

'Glass, indeed! Well, the explanation can wait – it's absolutely fascinating, but quite complicated. So I'll give you the straight facts.

'Mount Zeus is a single diamond, approximate mass one million million tons. Or, if you prefer it that way, about two times ten to the seventeenth carats. But I can't guarantee that it's all gem quality.'

VII – THE GREAT WALL

49 – Shrine

As they unloaded the equipment from Bill Tee and set it up on their little granite landing-pad, Chris Floyd found it hard to tear his eyes away from the mountain looming above them. A single diamond – bigger than Everest! Why, the scattered fragments lying round the shuttle must be worth billions, rather than millions.

On the other hand, they might be worth no more than – well, scraps of broken glass. The value of diamonds had always been controlled by the dealers and producers, but if a literal gem-mountain came suddenly on the market, prices would obviously collapse completely. Now Floyd began to understand why so many interested parties had focused their attention upon Europa; the political and economic ramifications were endless.

Now that he had at last proved his theory, van der Berg had become again the dedicated and single-minded scientist, anxious to complete his experiment with no further distraction. With Floyd's help – it was not easy to get some of the bulkier pieces of equipment out of Bill Tee's cramped cabin – they first drilled a metre-long core with a portable electric drill, and carried it carefully back to the shuttle.

Floyd would have had a different set of priorities, but he recognized that it made sense to do the harder tasks first. Not until they had laid out a seismograph array and erected a panoramic TV camera on a low, heavy tripod did van der Berg condescend to collect some of the incomputable riches lying all around them.

'At the very least,' he said, as he carefully selected some of the less lethal fragments, 'they'll make good souvenirs.'

'Unless Rosie's friends murder us to get them.'

Van der Berg looked sharply at his companion; he wondered how much Chris really knew – and how much, like all of them, he was guessing.

'Not worth their while, now that the secret's out. In about an hour's time, the Stock Exchange computers will be going crazy.'

'You bastard!' said Floyd, with admiration rather than rancour. 'So that's what your messsage was about.'

'There's no law that says a scientist shouldn't make a little profit on the side – but I'm leaving the sordid details to my friends on Earth. Honestly, I'm much more interested in the job we're doing here. Let me have that wrench, please...'

Three times before they had finished establishing Zeus Station they were almost knocked off their feet by quakes. They could feel them first as a vibration underfoot, then everything would start shaking – then there would be a horrible, long-drawn-out groaning sound that seemed to come from every direction. It was even air-borne, which to Floyd seemed strangest of all. He could not quite get used to the fact that there was enough atmosphere around them to allow short-range conversations without radio.

Van der Berg kept assuring him that the quakes were still quite harmless, but Floyd had learned never to put too much trust in experts. True, the geologist had just been proved spectacularly right; as he looked at Bill Tee heaving on its shock-absorbers like a storm-tossed ship, he hoped that Van's luck would hold for at least a few more minutes.

'That seems to be it,' said the scientist at last, to Floyd's great relief. 'Ganymede's getting good data on all channels. The batteries will last for years, with the solar panel to keep recharging them.'

'If this gear is still standing a week from now, I'll be very surprised. I'll swear that mountain's moved since we landed – let's get off before it falls on top of us.'

'I'm more worried,' laughed van der Berg, 'that your jet-blast will undo all our work.'

'No risk of that – we're well clear, and now we've offloaded so much junk we'll need only half-power to lift. Unless you want to take aboard a few more billions. Or trillions.'

'Let's not be greedy. Anyway, I can't even guess what this will be worth when we get it to Earth. The museums will grab most of it, of course. After that – who knows?'

Floyd's fingers were flying over the control panel as he exchanged messages with Galaxy.

'First stage of mission completed. Bill Tee ready for take-off. Flight plan as agreed.'

They were not surprised when Captain Laplace answered.

'You're quite certain you want to go ahead? Remember, you have the final decision. I'll back you up, whatever it is.'

'Yessir, we're both happy. We understand how the crew feels. And the scientific payoff could be enormous – we're both very excited.'

'Just a minute – we're still waiting for your report on Mount Zeus!'

Floyd looked at van der Berg, who shrugged his shoulders and then took the microphone.

'If we told you now, Captain, you'd think we were crazy – or pulling your leg. Please wait a couple of hours until we're back – with the evidence.'

'Hm. Not much point giving you an order, is it? Anyway – good luck. And from the owner as well – he thinks going to Tsien is a splendid idea.'

'I knew Sir Lawrence would approve,' Floyd remarked to his companion. 'And anyway – with Galaxy already a total loss, Bill Tee's not much extra risk, is it?'

Van der Berg could see his point of view, even though he did not entirely subscribe to it. He had made his scientific reputation; but he still looked forward to enjoying it.

'Oh – by the way,' Floyd said. 'Who was Lucy – anybody in particular?'

'Not as far as I know. We came across her in a computer search, and decided the name would make a good code word – everyone would assume it was something to do with Lucifer, which is just enough of a half-truth to be beautifully misleading....

'I'd never heard of them, but a hundred years ago there was a group of popular musicians with a very strange name – the Beatles – spelled B-E-A-T-L-E-S, don't ask me why. And they wrote a song with an equally strange title: "Lucy in the Sky with Diamonds". Weird, isn't it? Almost as if they knew...'

According to Ganymede radar, the wreck of the Tsien lay three hundred kilometres west of Mount Zeus, towards the twilight zone and the cold lands beyond. Permanently cold they were, but not dark; half the time they were brilliantly lit by the distant Sun. However, even by the end of the long Europan solar day, the temperature was still far below freezing point. As liquid water could exist only on the hemisphere facing Lucifer, the intermediate region was a place of continual storms, where rain and hail, sleet and snow contended for supremacy.