Now gaining, now falling behind, the midget sub marine stalked the nuclear vessel in the black waters of the lake. The skipper of the Thrush craft was adopting an erratic course, zigzagging from side to side of the reservoir, accelerating and slowing every few minutes.

"He's testing the surface maneuverability," Illya said. "Must be. I only wish we could surface too. It would be so much easier to keep track of him. But we don't dare - there might be a phosphorescent wake, a too-smart look out, anything. Well just have to hope he goes in soon."

Once, concentrating too closely on the livid radiance of the radar screens, they almost rammed the tiny craft straight into a massive wall of rock that rose straight from the valley floor. Another time, the nuclear sub marine took them by surprise making a tight U-turn and passing almost directly overhead in the opposite direction.

Looking up through their perspex domes at the faint hint of light drifting down from the surface, they watched the great hull - only inches above their eyes, it seemed - draw smoothly past. A shark shape, sinister and efficient, blotting out the light.

Shortly afterwards, there was a commotion in the water around them and the midget rocked violently. The big submarine was submerging.

"It crash-dived fifty fathoms," Illya exclaimed - it had taken them some time to locate it again on their screens - "and now it's scooting along the floor of the old valley for all it's worth!"

They followed the nuclear craft up to the far end of the dam and then back to the barrage again, where they "froze" near the rock face to minimize the risk of detection as the bigger vessel turned. "And off she goes again," Kuryakin said in exasperation. "But the power of that thing! Do you realize the speed she's doing on those straight runs? Why, on that last one, she was hitting -"

"Illya!" Coralie's voice was urgent over the intercom. "Be quiet a moment, will you? She's altered course through ninety degrees; she's turned off to port, towards the side of the dam. I think she may be going in."

"Okay. I'll try to catch up and tail her. At the last stage, I'll have to follow her by eye - for the only thing we can do is to follow her into this pen if possible - and hope that nobody spots us before they pump out the water!"

It was a strange journey, the last part of that under water voyage. Deeper and deeper they plunged, the black water streaming past the two inclined screens, the fragile craft vibrating with the thrust of its screws as the man and the girl bent over their screens concentrating on the luminous blob that represented the quarry they hunted. "We're at forty-three fathoms, Illya," Coralie warned anxiously. "The man said -"

"I can see her!" the Russian exclaimed suddenly. "Look!"

Ahead and below, the faintest hint of luminescence marbled the black. The radiance became discernible, stained the dark depths, wavered and spread, and finally revealed the cigar-shaped nose of their own craft, on which it then cast a discreet highlight. And in front of them, silhouetted against the underwater beam, the huge bulk of the submarine hung like a resting fish.

Put on your lung and harness" - Illya's voice was low - "and be ready to bale out at any time. No more talking after this."

As she shrugged into the shoulder straps and snapped the clasps about her hips, a series of rectangular planes assembled themselves in the faint illumination into the outline of some building. Monolithic and immense, it jutted from the drowned rock face like a legendary keep seen through a dream, its functional lines distorted and imprecise through the movement of the water. Some where near the top a great opening yawned - and it was from this gap in the façade that the light came streaming.

The submarine was moving again, the huge hull sliding quietly into the opening, the green light washing outwards contouring the sophisticated curves of its steel sides.

Kuryakin maneuvered the midget adroitly so that it was placed just behind and to one side of one of the vessel's rear quarters. Together the two ships, like a whale with an enemy pilot fish, sank into the gigantic underwater pen through the opening in the fortress wall. A moment later, colossal double doors rolled across and sealed off the entry. And then slowly, as air was pumped in at the top, the water level began to sink in the chamber.

Ten minutes later the nuclear submarine was resting against a dock on the left-hand side of the pen. The midget was submerged on its offside in the small amount of water that had been left in the chamber. And Illya and Coralie, breathing from the aqualungs on their backs, were just below the surface astern of the Thrush vessel, hoping that nobody would take it into his head to walk to the after rail and look directly down into the water.

Kuryakin raised a cautious head. Grotesquely distorted by the acoustics of the chamber, he heard the sound of feet and voices as the crew trooped ashore and waited their turn to go through the hydraulically controlled double doors leading to the interior of the fortress. There seemed to be no personnel on duty in the pen itself - and indeed why should there be, he thought, since it was really no more than an air-lock between the subterranean fortress and the lake?

When the last footstep had died away, he led the girl on a submerged exploration of the pen. There was about fifteen feet of water left in the chamber and the nuclear submarine was still just afloat. The place must be built, he guessed, directly onto some rocky prominence projecting through the lower parts of the fortress: with such a huge area, it would have been impossible to design a structure robust enough to tolerate all that weight of water if there had been other stories immediately below. Basically, the pen itself was just an enormous box, one end of which was formed by the watertight gates. Ceiling, floor, and wall and one side wall were unbroken by any projection or recess - and the remaining side wall, on the left as you entered through the gates, carried along its whole length the platform against which the vessel was moored. The surface of this quay was a couple of feet above the level of the water slapping and sucking at the submarine's sides. Above it, armored glass slits let in the green light which flooded the chamber.

Before they had quit the pen through the double doors at the far end of the quay, some of the crew had rolled ashore a quantity of steel drums which now lay neatly stacked near the craft's massive stern. Behind the shelter of these, Kuryakin reached up and gripped the edge of the platform to haul himself laboriously from the water. Flopping face down across the wet concrete, gasping, for a moment, he rose gingerly to his feet and held out his hands to Coralie.

They unhitched their aqualungs and propped them up against the drums, turning to survey the great pen now from above water level. The submarine filled exactly half the space available. Everywhere around them, above and on all sides, reminders of how recently the place had been simply an oversize tank obtruded on eye and ear. Moisture streamed down the blank walls, dropped hollowly to the curved decks of the ship from the roof, trickled into the water, and dripped from every ledge and cranny and beam and angle to be seen. The Russian had pulled off his helmet and was halfway out of his frogman suit when Coralie laid a hand on his arm.

"It's going to take an age to get back into these if they're still wet," she whispered. "Don't you think perhaps we should keep them on, just in case we have to leave this way in a hurry?"

Illya shook his head. "What you say is quite true," he said. "But look…" He pointed to the girl's legs.