"They've even got flippers.'

'Not bad, eh? Instead of sinking like a stone, you'll be swimming like a frogman.' Frost pointed to the numerous articulated joints. 'It's built to ensure complete freedom of movement, even at four hundred metres. Your hands are protected inside two pods – no articulated gloves, I'm afraid: the fingers would be too delicate. Instead you've got computer-operated manipulators on the end of each arm. The sensors provide tactile feedback for your hands inside the pods. They're incredibly sensitive – you could write your own will on the seabed if you wanted.'

'How long can we stay down?'

'Forty-eight hours,' said van Maarten. He saw the alarm on Bohrmann's face and grinned. 'Don't worry, you'll be finished long before then.' He pointed to two torpedo-shaped robots, each measuring roughly 1.5 metres. They were equipped with propellers, and their tips were encased in translucent plastic. Several metres of cable were attached to each robot, connecting them to a console with handles, a display and buttons. 'These are your trackhounds. AUVs. They're programmed to find the lighting scaffold, and they're accurate to within a few centimetres, so please don't attempt to find your own way. Just let yourselves be towed. They go at a rate of four knots, so you'll be there in three minutes.'

'How reliable are they?' Bohrmann enquired.

'Very. Trackhounds come equipped with all kinds of sensors that determine their position and their depth in the water. You're certainly not going to get lost, and if anything gets in the way, the trackhound will dodge it. They're activated via the console at the end of the leash. Descent, ascent – easy. The button marked zero starts the propeller without activating the navigational program, so you can steer the trackhound with the joystick instead. Your dog will scamper in whichever direction you choose. Any questions?'

Bohrmann shook his head.

'Let's go, then.'

Van Maarten helped them into the suits. Entry was via a flap in the back, to which two oxygen tanks were mounted. Bohrmann felt like a knight in full armour, about to take a stroll on the moon. As the suit closed, all went silent for a moment, then the volume returned. Through the visor he could see Frost talking to him from inside his own suit, and then the volcanologist's voice boomed into his ears. He could even hear outside noise.

'Wireless communication,' explained Frost. 'It's more reliable than hand signals. Are you getting the hang of the manipulators?'

Bohrmann wiggled his fingers inside the pod. The manipulator copied his movements. 'I think so.'

'Van Maarten's going to give you the console. Try to get hold of it.'

It worked the first time. Bohrmann gave a sigh of relief If everything was as easy as operating the manipulators, they would be fine.

'One more thing. If you look down at your suit, there's a raised rectangular panel. It's at waist level – like a flat switch. It's a POD.'

'A what?'

'Nothing you need worry about now. It's just a precaution. If we need them, I'll explain what they're for. To turn it on, you push it firmly. OK?'

'What is it?'

'A good thing to have when you're diving.'

'I'd really rather-'

I'll tell you later. All set?'

'All set.'

Van Maarten opened the hatch to the sluice tunnel. Lit up in the artificial light, the bright blue water sloshed towards them. 'Just topple in,' he said. I'll send the trackhounds after you. Don't switch them on until you're out of the tunnel. Stan, I'd suggest you start yours first.'

Bohrmann shuffled his flippers towards the edge. Even the tiniest movement was an amazing feat of strength. He took a deep breath and allowed himself to fall forwards. The water rose up towards him and he saw the artificial lights of the tunnel flash above him, then found himself upright. He sank slowly through the tunnel and out into the sea, landing in a shoal of fish. Thousands of shimmering bodies dispersed, regrouping in a tightly packed spiral. The shoal changed shape a few times, strung out in a line, and fled. Bohrmann saw the trackhound beside him and sank deeper. Above him the tunnel glowed against the dark contours of the pontoon. He kicked his fins and realised he could hover on the spot. The divesuit felt good now, like wearing his own submersible.

Frost followed him in a column of bubbles, then sank until he was on a level with Bohrmann and looked at him through the view port. Bohrmann saw that the American was still wearing his cap.

'How are you feeling?' asked Frost.

'Like R2-D2's older brother.'

Frost laughed. The propeller on his trackhound started to turn. Suddenly the robot dipped its nose and pulled the volcanologist into the depths. Bohrmann activated his own. He felt a sharp jerk, then shot off head-first. The water darkened. Van Maarten was right these things were fast. In no time it was pitch black. It was impossible to see anything apart from the diffuse rays of light emitted by the trackhounds.

To his surprise he felt uneasy in the darkness. He'd sat in front of the screen hundreds of times, watching robots dive to the abyssal plains or even as far as the benthic zone. He'd been down to a depth of four thousand metres in the legendary submersible Alvin, yet nothing had prepared him for being encased in a suit and whisked into the unknown by an electronic guide-dog.

Hopefully the thing he was clutching had been properly programmed or there was no telling where he was headed.

Showers of plankton appeared in the glow of the floodlights and the electronic hum of the trackhounds buzzed inside Bohrmann's helmet. Ahead he saw a delicate creature drifting through the night with elegant pulsing movements, a deep-sea jellyfish sending out ring-shaped signals of light like a spaceship. Bohrmann hoped they weren't being emitted in panic as it fled from some predator. Then the jellyfish disappeared. More jellyfish luminesced in the distance, and a bright cloud flashed before his eyes. He couldn't help flinching. But the cloud was white, not blue. Its source luminesced briefly before it disappeared within its own mist. Bohrmann knew what it was: a mastigoteuthid, or whiplash squid, a creature usually only found at depths of around a thousand metres. It made sense for it to expel white ink when threatened – in the darkness of the depths, black would be useless.

The dog strained at its leash.

Bohrmann scanned the water for a glimpse of the lighting scaffold, but he was surrounded by darkness, with just a faint dot of light moving in front of Frost. At least, he assumed it was moving. It might as well have been stationary: two fixed points of lights, his beam and Frost's, in a starless universe.

'Stanley?'

'What's up?'

The promptness of the answer soothed him.

'It's about time we saw something, isn't it?'

'You've got to be patient, buddy. Look at the display. We've only gone two hundred metres.'

'Oh. Of course. No problem.'

Bohrmann didn't dare ask Frost whether he was sure that the track-hounds had been properly programmed, so he kept quiet and tried to stifle his mounting anxiety. He almost wished a few more jellyfish would show themselves, but there was nothing to be seen. The robot hummed busily. All of a sudden Bohrmann felt a change of direction.

There was something ahead. Bohrmann screwed up his eyes and made out a distant glow. At first it was just a faint patch of light, then a hazy rectangle.

He could barely contain his relief Good dog, he felt like saying. There's a good boy.

How small the lighting scaffold looked.

He was still puzzling over its size as the distance decreased and the glow brightened, separating into individual floodlights along the unit's frame. They continued towards it, and suddenly it was above them, a canopy of light overhead. Of course, they were above and it was below, but the head-first dive had turned everything upside-down. Next the terrace appeared, and it, too, was suspended in the sky. For a moment Frost became visible, a shadow being pulled by a torpedo on a leash, rushing towards a football field of light. Now the view opened up before them: the terrace, the snake-like body of the tube towering out of the darkness, the lumps of rock blocking its mouth…