Twenty-five minutes later, the boys were sitting on a two-man rover that moved easily through the water. It was shaped like a squat torpedo and had two seats, one behind the other. As Marine Boy had predicted, Jack had found his diving gear neatly racked, though the leg of his suit had been shredded.

"I should be able to find one that'll fit you" Marine Boy had said, but Jack had told him not to bother, suggesting that the water was warm enough not to need one. Marine Boy had rather suspected that Jack had other reasons for choosing to wear nothing but a brief pair of trunks, but had been more than happy to go along.

Borrowing the rover had presented no problems, even if Jack suspected that Marine Boy was using the word "borrow" in an imaginative way. It was surprisingly fast, and Jack found the ride exhilarating. For balance (and other reasons) he wrapped his arms round Marine Boy's waist as they zipped along.

Eventually, Marine Boy slowed the rover. "This is where we found you," he said, looking over his shoulder at Jack. Jack nodded.

"Right: which way was that sub headed?"

Jack pointed, and Marine Boy turned the nose of the rover. Ahead was a maze of rocky outcrops; some insignificant, others the size of a large hill. "This area's not been properly charted," he said, "or given much more than a passing glance, really. All I can tell you is that it covers about ten square miles, and contains no mineral deposits worth talking about. Mind you, I bet it also contains any number of good hiding places. There could be a dozen subs in here and we'd be none the wiser."

Marine Boy steered the rover this way and that trying to be methodical, though the random nature of the rock formations made this a tall order. "This place needs a name," he said. "How about 'Jack's Rocks'?" He glanced behind him. Jack shook his head.

"No?" Marine Boy asked. "Yeah, you're right, I can do better. 'Templeton's Peaks'?"

Marine Boy waited for a response, but instead Jack tapped him urgently on the shoulder and pointed away to their right. "Seen something?" Marine Boy asked, turning the rover, "Oh yes, that could be worth checking out. Looks like the entrance to a cave. Well spotted."

The cave proved to be further off than expected, and larger. It was situated near the bottom of what, on land, would've been considered a fair-sized hill. They reached the cave mouth. It was dark, but Marine Boy flipped a switch and the rover's headlight pierced the gloom. The cave went back quite a long way. "Wow," Marine Boy said. Not so much a cave, more the entrance to a tunnel. I wonder how far it goes on? I mean look how it bends to the left in the distance. You could certainly get a sub in here – a small one anyway. Shall we?" Jack nodded.

Marine Boy coaxed the rover slowly into the tunnel, which sloped gently upwards. The walls were jagged and uneven. Marine Boy stared at them closely. "You know, I could almost swear that this tunnel is man-made, or that if it was natural to start with, it's been enlarged." He looked back at Jack, who gave a theatrical shrug. "Why do I think that?" Marine Boy asked. "Because there's so little life on the walls and floor – shellfish, sponges and stuff. If this cave – or tunnel or whatever – has been here millions of years, you'd expect a lot more marine life to have colonised it. Maybe I'm wrong, but this place feels fishy because it's not fishy enough." He turned and flashed a smile at Jack who, far from appreciating this shaft of wit, mimed a vomiting fit. "Please yourself" Marine Boy told him.

They reached the turn and began to head upwards into the heart of the hill. Then suddenly – and to the astonishment of them both - they broke surface in an air-filled cavern.

"Wow," Jack said taking the breathing tube from his mouth.

"Wow," Marine Boy echoed. He sniffed the air for a bit. "Man-made or what?"

"Unless you believe in mermaids, "Jack said."

"Funny you should mention Mermaids..." Marine Boy began.

"No way!" Jack said, his eyes opening wide. Then: "Oh, you're joking."

"Almost had you though. But let's see what we have here." The rover was equipped with a powerful hand-held torch. Marine Boy played it round the cavern. It was large but not high-ceilinged. Half of the area was taken up by a flat area, unquestionably man-made, just above the water level. Marine Boy handed the torch to Jack, and steered for the platform. On it stood a derrick-like structure, made from bolted steel.

"An undersea dock," Marine Boy said. He brought the rover alongside and cut the engine. It drifted to a stop. There were standard mooring bollards embedded in the edge of the platform, and Marine Boy made the rover fast before the two boys stepped onto the dock. "What do you suppose that's for?" he asked, indicating the steel structure.

"To set alongside the conning tower of a sub when it ties up here?" Jack suggested. "I mean, the size would be about right for the sub I saw, I reckon."

"You're probably right. A sub base, but no sub."

"Bet there was one, though. Ow, my ears just popped."

"Yes, well this place must be pressurised, to keep the water level down. We're not that deep, but there must be three atmospheres in here, if not four."

"Is that harmful?"

"Not in the short term. All the same, this is a pretty impressive piece of engineering."

"And one which Ocean Base never even suspected."

"No; and that's what worries me."

"I'll bet," Jack said. "So what do we do now – search for clues?"

"Such as what?"

"You're the expert." Jack continued to play the beam of the torch over the walls of the cavern. "I wonder what it's all for?"

"That's what we need to find out. Hey, swing the torch back where you just had it... left, left, down a bit... there. What do you make of that?"

"No way," said Jack.

"Yes way, if you ask me," Marine Boy said.

"A light switch? Down here?"

"Why not? People like to be able to see what they're doing, and compared to carving out this docking bay, I'd've thought installing lighting was no big deal." He started to walk over to it.

"You're not going to flip it, are you?" Jack asked him.

"I certainly am." The switch was bigger than any domestic one, but its purpose seemed clear to Marine Boy. He flipped it. A dozen large lamps attached to the roof of the cave blinked themselves on, filling the place with dazzling white light.

"Wow," both boys said together.

"There's even more to this place than I thought," Marine Boy said, after looking about him.

"Lucky for us no-one's around."

"You sure?"

"Can you see anyone?"

"No, but there are other things I can't see. Like a generator. These lights don't run on seawater, you know. And the air's been filtered and purified – trust me on this – but I don't see any filtration units. So where are they, and the generator?"

"Under water?"

"I doubt it. Water and electricity don't mix very well. It stands to reason that whoever built this place would've put the generator somewhere dry if they could."

"So where is it?" asked Jack.

"Clearly we're not seeing all of this place, which is why I said there was more to it than we thought. Best guess – some of this rock wall isn't what it seems, and conceals a doorway of some sort."

"A secret door? Why?"

"I said best guess. As for why, that's a mystery."

"I don't see any door-handle, or a bell, or a welcome mat," Jack said. "And I still reckon no-one's at home."

"What makes you so sure?"

"Well, we've not been that subtle, have we? I mean, sailing straight in, turning the lights on and so forth. Anyone secretive enough to disguise a door as a lump of rock is probably going to have a burglar alarm as well, and I bet we've already tripped it. But no-one's turned up to ask awkward questions. In other words, the fact that no-one has yet come charging in with guns and harpoons suggests that there's no-one at home."

"Brilliant, Holmes."

"Elementary, my dear Johnny."

"And can your massive brain work out where they've got to?"

"They're probably all off in their sub – though where that might be is anyone's guess."

"You mean there's something you can't deduce? Disappointing."

"Nobody's perfect. So – what do we do now?"

"There's much we can do – other than go back and report all this."

"Sounds a bit of an anti-climax."

"Life is, sometimes."

"I guess. We don't have to go back right now, do we?"

"You mean lie in wait and see who turns up? I mean they're bound to sooner or later. Whoever built this place hasn't abandoned it."

"Maybe, but that wasn't what I had in mind."

"Wasn't it?" Marine Boy asked. "What, then?"

"Oh, I don't know. You and me, alone, with all the privacy we could possibly want..."

"What do you... oh. Oh..."

"Exactly. If you're in the mood, that is. You can always say no."

"Fat chance. You know, it almost feels as though you planned this."

"Oh sure," said Jack, sarcastically, "I hollowed this entire place out myself and installed all the workings. After that I got myself rescued by great Marine Boy, then lured him here with a story about a sub. And all because I wanted somewhere quiet to fondle his arse."

"Yeah, perhaps not."

"It would be worth it to go to all that trouble, though," Jack said. "I mean, you do have the cutest arse – particularly when you're wearing that nice tight suit."

"Yours isn't so bad, either. In fact, all of you is scrummy, especially when you're wearing practically nothing."

"Is that so?" asked Jack, "shrugging off his scuba tank and undoing his weight belt. "So make my day."

Marine Boy walked up to Jack and kissed him. He found that he had to put his arms round Jack's neck and raise himself on his toes to reach the mouth of the taller boy. Jack put one hand in the small of Marine Boy's back and pulled him close. With the other he reached down and squeezed.

Marine Boy squirmed pleasurably in Jack's grip. "God, that feels good," he said.

"Sure does," Jack told him.

"Mmm. Can you, er, rub me a bit harder, please? Oh, yes!"

"I must say I love your suit," Jack said, when Marine Boy allowed him to breathe. "Skin-tight and smooth as silk. Makes me want to run my hands all over you. And I mean all over."

"And I want to lick every bit of you. Every bit. I wish we could lie down, though. Cold hard concrete's a bit of a passion-killer."

"You have a point," Jack said. "Hey, do you want to try something else instead?"

"You bet."

"You haven't asked what."

"Don't need to, you have such great ideas. Whatever you've thought of will be awesome. Do you want me to give you that blow-job I owe you? 'Cos I'm ready."

"Maybe later. And for crying out loud, stop saying you owe me. I already told you that sharing means you don't have to keep count."

"Sorry."

"So you should be. If you say it again, I might have to put you over my knee."

"Mmm..."

"What? Oh you're impossible. One thing at a time, for chrissakes."

"You're the boss. What do you have in mind?"

Jack walked over to where a coil of rope lay on the floor. With his diving knife he cut off a few lengths and held them up for Marine Boy's inspection. "You know you said you might like it if someone you trusted was to tie you up and, er, so forth?" He nodded towards the derrick. "Well how about it?"