And then Pitt saw it; a black and ominous opening just twelve feet below the surface. The size was perfect; big enough for a submarine or, for that matter, a locomotive to have been driven in. They all hung suspended in the clear crystal water, their eyes fixed on the cave entrance, hesitating, exchanging glances.

Pitt moved first, entering the hole. Except for a few dim flashes of light, reflected from the whites of his heels, he disappeared completely from view, swallowed by the yawning cavity.

He leisurely beat the water with his fins and let an incoming swell help carry him slowly through the tunnel. The bright blue-green of the sunlit sea rapidly transformed into a kind of deep twilight blue. At first Pitt could see nothing, but soon his eyes adjusted to the dark interior, and he began to make out a few details of his surroundings.

There should have been a myriad of marine life clinging to the tunnel walls. There should have been darting crabs, winking limpets and barnacles, or crawling lobsters, sneaking about in search of tasty shellfish. There were none of these. The rocky sides were barren, and they were coated with a reddish substance that clouded the water whenever Pitt touched the smooth, unnatural material. He rolled face up and inspected the arched roof, watching in fascinated interest as his exhaust bubbles rose and wandered across the ceiling, like a trail of quicksilver, seeking escape from a vial.

Abruptly the roof angled upward, and Pitt’s head broke the surface. He looked around but saw nothing; a gray cloud of mist obscured everything. Puzzled, he ducked his head back in the water and dove, leveling out at ten feet. Beneath him a cylindrical shaft of cobalt light flowed in from the tunnel. The water was as clear as air; Pitt could see every nook and cranny of the cavern's submerged area.

An aquarium. That was the only way Pitt could describe it. But for the fact that there were no portholes in the walls, the cavern could have easily passed for the main tank at Marineland in California. It was a far cry from the tunnel; marine life abounded everywhere. The lobsters were here, and so were the crabs, the limpets, the barnacles, even a heavy growth of kelp. There were also roving schools of brilliantly colored fish. One fish in particular caught Pitt’s eye, but before he could get closer, it saw his approach and flashed into a protective rock fissure.

For several moments, Pitt took in the breathtaking scene. Then suddenly, he started as a foreign hand grabbed his leg. It was Ken Knight, and he was motioning toward the surface. Pitt nodded and swam to the top. Again he was greeted by the heavy mist.

Pitt spit out his mouthpiece. “What do you make of it?” he asked. The rock walls amplified his voice to a roar.

“A fairly common occurrence,” Knight answered, roaring back matter of factly. “Every time a swell hits the entrance outside, the force runs like a piston through the tunnel, compressing the air already trapped in the cavern. When the pressure recedes, the expanded, moisturized air cools and condenses in a fine mist.” Knight paused to blow some mucus from his nose. “The swells are running at about twelve-second intervals, so it should start to clear up at any time.”

No sooner had he said it than the mist disappeared, revealing a dim cavern that arched to a dome sixty feet overhead. It was a drowned grotto and nothing more; no traces of man-made equipment. Pitt felt as though he had entered a deserted cathedral whose spires stood in ruined desolation from a World War I artillery shelling or a World War II aerial bombardment The walls were twisted and broken in jagged fissures, and. the shattered rocks at their base showed that another rock fall could come at any time.

Then the mist returned and smothered all vision.

Pitt, in the few seconds it took to survey the cavern, was conscious of nothing but the gnawing fear of self-doubt. Then came a creeping wave of numbed disbelief, then the chagrin that he had bungled it.

"It can’t be,” he muttered. “It just can’t be.” Pitt’s free hand curled into a white knuckled fist, and he pounded the water in an outburst of temper and despair. “This cavern had to be von Till’s base of operations. God help us from the mess that I’ve surely caused.”.

“I'd still vote for you, Major,” Knight reached out and touched Pitt on the shoulder. “The geology bears out your hunch. This would seem the most logical spot.”

“It’s a dead end. Except for the tunnel, there’s no openings, anywhere.”

“I saw a ledge on the far end of the cave. Maybe if

I—”

“No time for that,” Pitt interrupted impatiently. “We must get back out as fast as we can and keep searching.”

“Excuse me, Major!” Hersong had caught Pitt’s arm, an action that surprised Pitt by seemingly coming out of nowhere. “I found something that might be of interest.”

The mist went through its cycle and then cleared again, revealing a peculiar expression on Hersong’s face that caught Pitt’s attention. He grinned at the lanky botanist.

“OK, Hersong. let’s make it quick. We hardly have time for a lecture on marine flora.”

“Believe it or not, that’s just what I had in mind,” Hersong grinned back; the glistening water trickled through the strands of his red beard. “Tell me, did you notice that growth of Macrocystis pyrifera on the wall opposite the tunnel?”

“I might have,” Pitt answered flatly, “if I knew what you were talking about”

"Macrocystis pyrifera is a brown algae of the Phaeophyta family, perhaps, better known as kelp.”

Pitt stared at him, considering, and let him continue.

“What it boils down to, Major, is that this particular species of kelp is native only to the Pacific Coast of the United States. The water temperature in this part of the Mediterranean is far too warm for Macrocystis pyrifera to survive. On top of that, kelp, like in land plant cousins, needs sunlight to provide the process for photosynthesis. I can’t imagine kelp thriving in an underwater cave. Nope, if you’ll forgive the vernacular, it just ain’t done.”

Pitt was slowly treading water. “Then if it isn’t kelp, what is it?”.

The mist was back, and Pitt couldn’t see Hersong’s face. He could only hear the botanists rumbling voice.

“It’s art, Major. pure art. Without a doubt, the finest plastic replica of Macrocystis pyrifera I’ve ever beheld.”

“Plastic?” Knight boomed, his tone echoing around the cavern. “Are you sure?”

“My dear boy,” Hersong said disdainfully. “Do I question your. analysis of’ core samples or—”

“That red slime on the tunnel walls,” Pitt cut in. “What do you make of that?”

“Couldn’t say for sure,” Hersong said. “Looked like some type of’ paint or coating.”

“I’ll back him. Major.” The face of Stun Thomas suddenly materialized out of the fading mist. “Red anti-fouling paint for ship hulls. It contains arsenic; that’s why nothing grows in the tunnel.”

Pitt glanced at his watch. ‘Time is running out.

This must be the place.”

“Another tunnel behind the kelp? Knight asked in a careful sort of voice. “Is that it, Major?”

“It’s beginning to look encouraging.” Pitt said quietly. “A camouflaged second tunnel that leads to a second cavern. Now I can see why von Till’s operation was never discovered by any native of Thasos.”

“Well,” Hersong purged the water from his mouthpiece. “I guess we keep going.”

“We have no other option,” Pitt said. “Are we all ready for another go?”

“All present and accounted for, except for Woodson,” Spencer answered.

Suddenly, at that instant, a flashbulb flooded the cavern in a bright blue light

“Nobody smiled,” Woodson observed sourly. He had drifted off to the far wall of the cavern, trying for the widest possible lens angle.

“Next time, yell sex.” Spencer joked back.

“It wouldn’t matter,” Woodson grunted. “None of you know what it means anyway.”