And the writhing mass of worms.

'Turn off your trackhound before you crash into those lights,' said Frost. 'We can swim the last few metres.'

Bohrmann flexed the fingers of his free hand and tried to get the manipulator to hit the right button. The first attempt failed, and he sped past Frost, who'd slowed down.

'Gairhard? Where do you think you're going?'

He tried again. The manipulator slipped. Finally he succeeded, kicked his fins a few times and realigned himself on the horizontal. The scaffold was very close, stretching out seemingly endlessly in all directions. After a few seconds Bohrmann recovered his sense of up and down, and the scaffold and the terrace were beneath him.

Kicking evenly, he swam to the wedged tube and sank alongside it. The scaffold was now fifteen metres above his head. Within an instant the worms were swarming over his fins. He had to force himself to ignore them. They didn't stand a chance against the suit. They were revolting, of course, but no more. Worms could never pose a danger to a creature of his size.

Or could they? After all, these worms weren't even meant to exist. The trackhound had sunk on to the terrace alongside him. Bohrmann parked it on a ledge of rock and looked up at the tube. Man-sized chunks of black lava blocked the propellers – nothing they couldn't handle, though. More worrying was the larger splinter of lava that was squashing the tube against the side. It looked at least four metres high. Bohrmann doubted that he and Frost would be able to shift it, even though things weighed less under water and lava was porous and relatively light.

Frost joined him. 'Disgusting,' he said. 'Those sons of Lucifer are everywhere.'

'What's everywhere?'

'Worms of course! I suggest we deal with the smaller chunks first and see how far we get. Van Maarten?' he called.

'Over.' There was a tinny quality to the man's voice. Bohrmann had forgotten that they could communicate with him, too.

'We're going to tidy up a bit down here. We'll start by clearing the propellers. If we're lucky, the tube might be able to work its own way free.'

'OK. Are you all right, Dr Bohrmann?'

'Never been better.'

Frost pointed to an almost spherical chunk of lava that was blocking the swivel joint of one of the propellers. 'We'll start with that.'

They got to work, and after a good deal of pushing and shoving, the rock came unstuck, freeing the propeller and squashing hundreds of worms.

'OK,' said Frost.

They moved two more boulders, but the next was larger. After a concerted effort they tipped it to one side.

'See how strong we are down here,' said Frost, enthusiastically. 'OK, Jan,' he said to van Maarten, 'we've only got one propeller to go. They don't look damaged. Can you rotate them? Don't turn them on, just rotate them.'

After a few seconds, the tube started to purr. One of the turbines was rotating on its shaft. Then the others began to turn.

'Good,' shouted Frost. 'Now try to switch them on.'

Having retreated to a safe distance a few metres away from the tube, they watched the propellers start up.

The tube juddered.

'No go,' said van Maarten.

'I can see that.' Frost scowled. 'Turn them the other way.'

That didn't work either, and silt was being churned up, making the water murkier by the second.

'Stop!' Bohrmann waved his segmented arms about. 'Hey guys, that's enough now! There's no point. You're only getting mud in our eyes.'

The propellers slowed to a halt. The cloud of silt dispersed, leaving muddy streaks in the water. They could barely make out the mouth of the tube.

'Great.' Frost opened a flat box on the side of his exosuit and took out two pencil-sized objects. 'That huge chunk of rock is what's causing the problems. I know you're not going to like this, Gairhard, but we're going to have to blow the damn thing up.'

Bohrmann's gaze shifted to the worms. They were rapidly reclaiming the freshly vacuumed terrace. 'It's a big risk,' he said.

'We'll use a small charge. We'll place it at the bottom of the rock, where the tip's digging into the terrace – blast its legs off, so to speak.'

Bohrmann pushed off, floating a metre or so upwards, then heading for the rock. It got muddier and murkier as he approached. He switched on his head torch and sank into the cloud of sediment. He lowered himself carefully, dropped on to his knees and manoeuvred his helmet as close as possible to the place where the rock was embedded in the ground. He used his two manipulators to sweep away the worms. Some lunged at him and tried to bite the articulated limbs. Bohrmann shook them away and examined the sediment. He found thin veins of dirty white hydrate. When he poked at them with the manipulators, the surrounding lava splintered and tiny bubbles spun towards him.

'No,' he said. 'Bad idea.'

'Do you have a better one?'

'Yes. We'll use more of the explosive, look for dents or cracks in the lower third of the boulder, and blow it up from there. With a bit of luck the top will fall off and we won't disturb the terrace beneath it.'

'OK.'

Frost swam through the cloud towards him. They rose up a little, and visibility improved. Working systematically, they searched the rock for suitable spots. Eventually Frost found a deep groove in the lava and filled it with something that looked like firm grey Plasticine. He poked a pencil-thin cylinder inside it.

'That should do the trick,' he said. 'Expect some flying debris. Let's get out of the way.'

They started up their trackhounds and hitched a ride to the edge of the illuminated zone where, after a few metres, the terrace ended in darkness. The shower of particles wasn't too bad there, so the light waves weren't being deflected by algae or other floating matter, yet the transition into darkness was abrupt. Light disappeared under water in a sequence determined by its wavelength – first red, after two or three metres, orange, then yellow. After ten metres only green and blue were left, until they, too, were absorbed or scattered as the water swallowed any vestige of light. After that the world ceased to exist.

Bohrmann was reluctant to venture from the relative safety of the illuminated zone into nothingness. He noticed with relief that Frost didn't appear to think that they needed to retreat any further. At the edge of the gloom, where the blue gave way to inky black, he could see what appeared to be a crevice in the flank. Maybe it was a cave. He imagined how the stone had tumbled into the depths in a stream of red hot lava, slowly cooling and setting in curious shapes. Suddenly he felt cold inside his suit – cold at the thought of spending a lifetime in the depths.

He looked up towards the lighting scaffold. There was nothing to be seen apart from a blue aura around the white floodlights.

'OK,' said Frost. 'Let's get this done with.' He activated the fuse.

A torrent of bubbles poured forth from the rock, mixed with splinters and lava dust. There was a rumbling noise inside Bohrmann's helmet. A dark ring spread outwards, followed by more bubbles, as the debris dispersed in all directions.

He held his breath.

Slowly, very slowly, the top half of the rock began to topple.

'Yes!' shouted Frost. 'Thank you, God!'

The rock was tipping faster now, pulled over by its own weight. It broke half-way down, dropping on to the terrace next to the pipe and creating another, larger cloud of sediment. Despite his body armour, Frost managed to jump up and down and waggle his arms. He looked like Neil Armstrong taking a giant leap for America on the surface of the moon.

'Hallelujah! Hey, van Maarten! We knocked the damn thing down. Give the tube another try!'

Bohrmann hoped with all his heart that the explosion wouldn't result in any more landslides. Through the swirling sediment he heard the propellers start up, and suddenly the tube moved. It crinkled up, then its far end rose like the head of a gigantic worm, lifting slowly out of the cloud. The mouth swivelled round, pointing straight at them, then turned in the other direction, as though it were surveying its surroundings. If Bohrmann hadn't known better, he would have thought they were done for.