From the speeding Gyrocar, Lucky caught glimpses of the mighty dome overhead. A city built under water, he reflected, requires engineering miracles to be practical.

There were domed cities in many places in the solar system. The oldest and most famous were on Mars. But on Mars, gravity was only two fifths of Earth normal, and pressing down on the Martian domes was only a rarefied, wispy atmosphere.

Here on Venus, gravity was five sixths Earth normal, and the Venusian domes were topped with water. Even though the domes were built in shallow sea so that their tops nearly broke surface at low tide, it was still a matter of supporting millions of tons of water.

Lucky, like most Earthmen (and Venusians, too, for that matter), tended to take such achievements of mankind for granted. But now, with Lou Evans returned to confinement and the problem involving him momentarily dismissed, Lucky's agile mind was putting thoughts together and craving knowledge on this new matter.

He said, "How is the dome supported, Dr. Morriss?"

The fat Venusian had recovered some of his composure. The gyrocar he was driving hurtled toward the threatened sector. His words were still tight and grim.

He said, "Diamagnetic force fields in steel housings. It looks as though steel beams are supporting the dome, but that's not so. Steel just isn't strong enough. It's the force fields that do it."

Lucky looked down at the city streets below, filled with people and life. He said, "Have there ever been any accidents of this type before?"

Morriss groaned, "Great space, not like this... We'll be there in five minutes."

"Are any precautions taken against accidents?" Lucky went on stolidly.

"Of course there are. We have a system of alarms and automatic field adjusters that are as foolproof as we can manage. And the whole city is built in segments. Any local failure in the dome brings down sections of transite, backed by subsidiary fields."

"Then the city won't be destroyed, even if the ocean is let in. Is that right? And this is well known to the populations?"

"Certainly. The people know they're protected, but still, man, a good part of the city will be rained. There's bound to be some loss of life, and property damage will be terrific. Worse still, if men can be controlled into doing this once, they can be controlled into doing it again."

Bigman, the third man in the gyrocar, stared anxiously at Lucky. The tall Earthman was abstracted, and his brows were knit into a hard frown.

Then Morriss grunted, "Here we are!" The car decelerated rapidly to a jarring halt.

Bigman's watch said two-fifteen, but that meant nothing. Venus's night was eighteen hours long, and here under the dome there was neither day nor night.

Artificial lights blossomed now as they always did. Buildings loomed clearly as always. If the city seemed different in any way, it was in the actions of its inhabitants. They were swirling out of the various sections of the city. News of the crisis had spread by the mysterious magic of word of mouth, and they were flocking to see the sight, morbidly curious, as though going to a show or a circus parade, or as men on Earth would flock for seats at a magnetonic concert.

Police held back the rumbling crowds and beat out a path for Morriss and the two with him. Already a thick partition of cloudy transite had moved down, blocking off the section of the city that was threatened by deluge.

Morriss shepherded Lucky and Bigman through a large door. The noise of the crowd muffled and faded behind them. Inside the building a man stepped hastily toward Morriss.

"Dr. Morriss..." he began.

Morriss looked up and snapped out hasty introductions. "Lyman Turner, chief engineer. David Starr of the Council. Bigman Jones."

Then, at some signal from another part of the room, he dashed off, his heavy body making surprising speed. He called out over his shoulder as he started, "Turner will take care of you two."

Turner yelled,, "Just a minute, Dr. Morriss!" but the yell went unheard.

Lucky gestured to Bigman, and the little Martian raced after the Venusian councilman.

"Is he going to bring Dr. Morriss back?" asked Turner worriedly, stroking a rectangular box he carried suspended from a strap over one shoulder. He had a gaunt face and red-brown hair, a prominently hooked nose, a scattering of freckles, and a wide mouth. There was trouble in his face.

"No," said Lucky. "Morriss may be needed out there. I just gave my friend the high sign to stick closely to him."

"I don't know what good that will do," muttered the engineer. "I don't know what good anything will do." He put a cigarette to his mouth and absently held one out to Lucky. Lucky's refusal went unnoticed for a few moments, and Turner stood there, holding the plastic container of smokes at arm's length, lost in a thoughtful world of his own.

Lucky said, "They're evacuating the threatened sector, I suppose?"

Turner took back his cigarettes with a start, then puffed strongly at the one between his lips. He dropped it and pressed it out with the sole of his shoe.

"They are," he said, "but I don't know..." and his voice faded out.

Lucky said, "The partition is safely across the city, isn't it?"

"Yes, yes," muttered the engineer.

Lucky waited a moment, then said, "But you're not satisfied. What is it you were trying to tell Dr. Morriss?"

The engineer looked hastily at Lucky, hitched at the black box he carried and said, "Nothing. Forget it."

They were off by themselves in a corner of the room. Men were entering now, dressed in pressure suits with the helmets removed, mopping perspiring foreheads. Parts of sentences drifted to their ears:

"... not more than three thousand people left. We're using all the interlocks now..."

"... can't get to him. Tried everything. His wife is on the etherics now, pleading with him..."

"Darn it, he's got the lever in his hand. All he has to do is pull it and we're..."

"If we could only get close enough to blast him down! If we were only sure he wouldn't see us first and..."

Turner seemed to listen to all of it with a grisly fascination, but he remained in the corner. He lit another cigarette and ground it out.

He burst out savagely, "Look at that crowd out there. It's fun to them. Excitement! I don't know what to do. I tell you, I don't." He hitched the black box he carried into a more comfortable position and held it close.

"What is that?" asked Lucky peremptorily. Turner looked down, stared at the box as though he were seeing it for the first time, then said, "It's my computer. A special portable model I designed myself." For a moment pride drowned the worry in his voice. "There's not another one in the galaxy like it. I always carry it around. That's how I know-..." And he

stopped again.

Lucky said in a hard voice. "All right, Turner, what do you know? I want you to start talking. Now!"

The young councilman's hand came lightly to rest upon the engineer's shoulder, and then his grip began to tighten just a bit.

Turner looked up, startled, and the other's calm, brown eyes held him. "What's your name again?" he said.

"I'm David Starr."

Turner's eyes brightened. "The man they call 'Lucky' Starr?"

"That's right."

"All right, then, I'll tell you, but I can't talk loudly. It's dangerous."

He began whispering, and Lucky's head bent toward him. Both were completely disregarded by the busily hurrying men who entered and left the room.

Turner's low words flooded out now as though he were glad to be able to get rid of them. He said, "The walls of the city dome are double, see. Each wall is made of transite, which is the toughest, strongest sili-cone plastic known to science. And it's backed by force beams. It can stand immense pressures. It's completely insoluble. It doesn't etch. No form of life will grow on it. It won't change chemically as a result of anything in the Venusian ocean. In between the two parts of the double wall is compressed carbon dioxide. That serves to break the shock wave if the outer wall should give way, and of course the inner wall is strong enough to hold the water by itself. Finally, there's a honeycomb of partitions between the walls so that only small portions of the in-between will be flooded in case of any break."

"It's an elaborate system," said Lucky.

"Too elaborate," said Turner bitterly. "An earthquake, or a Venusquake, rather, might split the dome in two, but nothing else can touch it. And there are no Venus-quakes in this part of the planet." He stopped to light still another cigarette. His hands were trembling. "What's more, every square foot of the dome is wired to instruments that continually measure the humidity between the walls. The slightest crack anywhere and the needles of those instruments jump. Even if the crack is microscopic and completely invisible, they jump. Then bells ring and sirens sound. Everyone yells, 'Beware water!'"

He grinned crookedly. "Beware water! That's a laugh. I've been on the job ten years, and in all that time the instruments registered only five times. In every case repairs took less than an hour. You phi a diving bell on the affected part of the dome, pump out the water, fuse the transite, add another gob of the stuff, let it cool. After that, the dome is stronger than before. Beware water! We've never had even a drop leak through."

Lucky said, "I get the picture. Now get to the point"

"The point is overconfidence, Mr. Starr. We've partitioned off the dangerous sector, but how strong is the partition? We always counted on the outer wall's going gradually, springing a small leak. The water would trickle in, and we always knew that we would have plenty of time to get ready for it. No one ever thought that someday a lock might be opened wide. The water will come in like a fat steel bar moving a mile a second. It will hit the sectional transite barrier like a spaceship at full acceleration."

"You mean it won't hold?"

"I mean no one has ever worked out the problem. No one has ever computed the forces involved-until half an hour ago. Then I did, just to occupy my time while all this is going on. I had my computer. I always have it with me. So I made a few assumptions and went to work."

"And it won't hold?"

"I'm not certain. I don't know how good some of my assumptions are, but I think it won't hold. I think it won't. So what do we do? If the barrier doesn't hold, Aphrodite is done. The whole city. You and I and a quarter of a million people. Everybody. Those crowds outside that are so excited and thrilled are doomed once that man's hand pulls downward on the switch it holds."

Lucky was staring at the man with horror. "How long have you known this?"

The engineer blurted in immediate self-defense,

"Half an hour. But what can I do? We can't put subsea suits on a quarter of a million people! I was thinking of talking to Morriss and maybe getting some of the important people in town protected, or some of the women and children. I wouldn't know how to pick which ones to save, but maybe something should be done. What do you think?"

"I'm not sure."

The engineer went on, harrowed. "I thought maybe I could put on a suit and get out of here. Get out of the city altogether. There won't be proper guards at the exits at a time like this."

Lucky backed away from the quivering engineer, his eyes narrowed. "Great Galaxy! I've been blind!"

And he turned and dashed out of the room, his mind tingled with a desperate thought.