"Take it easy, Jack," he said mildly-it was the first time he had used my first name; I noted it and discounted it. "Nobody's going to censor you. We're glad to cooperate with you fellows, but the Moon's had too much bad publicity now-publicity it didn't deserve."

I didn't say anything.

"Every engineering job has its own hazards," he insisted, "and its advantages, too. Our men don't get malaria and they don't have to watch out for rattlesnakes. I can show you figures that prove it's safer to be a sandhog in the Moon than it is to be a file clerk in Des Moines-all things considered. For example, we rarely have any broken bones in the Moon; the gravity is so low-while that Des Moines file clerk takes his life in his hands every time he steps in or out of his bathtub."

"Okay, okay," I interrupted, "so the place is safe. What's the catch?"

"It is safe. Not company figures, mind you, nor Luna City Chamber of Commerce, but Lloyd's of London."

"So you keep unnecessary airlocks. Why?"

He hesitated before he answered, "Quakes."

Quakes. Earthquakes-moonquakes, I mean. I glanced at the curving walls sliding past and I wished I were in Des Moines. Nobody wants to be buried alive, but to have it happen in the Moon-why, you wouldn't stand a chance. No matter how quick they got to you, your lungs would be ruptured. No air.

"They don't happen very often," Knowles went on, "but we have to be prepared. Remember, the Earth is eighty times the mass of the Moon, so the tidal stresses here are eighty times as great as the Moon's effect on Earth tides."

"Come again," I said. "There isn't any water on the Moon. How can there be tides?"

"You don't have to have water to have tidal stresses. Don't worry about it; just accept it. What you get is unbalanced stresses. They can cause quakes."

I nodded. "I see. Since everything in the Moon has to be sealed airtight, you've got to watch out for quakes. These airlocks are to confine your losses." I started visualizing myself as one of the losses.

"Yes and no. The airlocks would limit an accident all right, if there was one-which there won't be-this place is safe. Primarily they let us work on a section of the tunnel at no pressure without disturbing the rest of it. But they are more than that; each one is a temporary expansion joint. You can tie a compact structure together and let it ride out a quake, but a thing as long as this tunnel has to give, or it will spring a leak. A flexible seal is hard to accomplish in the Moon."

"What's wrong with rubber?" I demanded. I was feeling jumpy enough to be argumentative. "I've got a ground-car back home with two hundred thousand miles on it, yet I've never touched the tires since they were sealed up in Detroit."

Knowles sighed. "I should have brought one of the engineers along, Jack. The volatiles that keep rubbers soft tend to boil away in vacuum and the stuff gets stiff. Same for the flexible plastics. When you expose them to low temperature as well they get brittle as eggshells."

The scooter stopped as Knowles was speaking and we got off just in time to meet half a dozen men coming out of the next airlock. They were wearing spacesuits, or, more properly, pressure suits, for they had hose connections instead of oxygen bottles, and no sun visors. Their helmets were thrown back and each man had his head pushed through the opened zipper in the front of his suit, giving him a curiously two headed look. Knowles called out, "Hey, Konski!"

One of the men turned around. He must have been six feet two and fat for his size. I guessed him at three hundred pounds, earthside. "It's Mr. Knowles," he said happily. "Don't tell me I've gotten a raise."

"You're making too much money now, Fatso. Shake hands with Jack Arnold. Jack, this is Fatso Konski-the best sandhog in four planets."

"Only four?" inquired Konski. He slid his right arm out of his suit and stuck his bare hand into mine. I said I was glad to meet him and tried to get my hand back before he mangled It.

"Jack Arnold wants to see how you seal these tunnels," Knowles went on. "Come along with us."

Konski stared at the overhead. "Well, now that you mention it, Mr. Knowles, I've just finished my shift."

Knowles said, "Fatso, you're a money grubber and inhospitable as well. Okay-time-and-a-half." Konski turned and started unsealing the airlock.

The tunnel beyond looked much the same as the section we had left except that there were no scooter tracks and the lights were temporary, rigged on extensions. A couple of hundred feet away the tunnel was blocked by a bulkhead with a circular door in it. The fat man followed my glance. "That's the movable lock," he explained. "No air beyond it. We excavate just ahead of it."

"Can I see where you've been digging?"

"Not without we go back and get you a suit."

I shook my head. There were perhaps a dozen bladder-like objects in the tunnel, the size and shape of toy balloons. They seemed to displace exactly their own weight of air; they floated without displaying much tendency to rise or settle. Konski batted one out of his way and answered me before I could ask. "This piece of tunnel was pressurized today," he told me. "These tag-alongs search out stray leaks. They're sticky inside. They get sucked up against a leak, break, and the goo gets sucked in, freezes and seals the leak."

"Is that a permanent repair?" I wanted to know.

"Are you kidding? It just shows the follow-up man where to weld."

"Show him a flexible joint," Knowles directed.

"Coming up." We paused half-way down the tunnel and Konski pointed to a ring segment that ran completely around the tubular tunnel. "We put in a flex joint every hundred feet. It's glass cloth, gasketed onto the two steel sections it joins. Gives the tunnel a certain amount of springiness."

"Glass cloth? To make an airtight seal?" I objected.

"The cloth doesn't seal; it's for strength. You got ten layers of cloth, with a silicone grease spread between the layers. It gradually goes bad, from the outside in, but it'll hold five years or more before you have to put on another coat."

I asked Konski how he liked his job, thinking I might get some story. He shrugged. "It's all right. Nothing to it. Only one atmosphere of pressure. Now you take when I was working under the Hudson-"

"And getting paid a tenth of what you get here," put in Knowles.

"Mr. Knowles, you grieve me," Konski protested. "It ain't the money; it's the art of the matter. Take Venus. They pay as well on Venus and a man has to be on his toes. The muck is so loose you have to freeze it. It takes real caisson men to work there. Half of these punks here are just miners; a case of the bends would scare 'em silly."

"Tell him why you left Venus, Fatso."

Konski expressed dignity. "Shall we examine the movable shield, gentlemen?" he asked.

We puttered around a while longer and I was ready to go back. There wasn't much to see, and the more I saw of the place the less I liked it. Konski was undogging the door of the airlock leading back when something happened.

I was down on my hands and knees and the place was pitch dark. Maybe I screamed-I don't know. There was a ringing in my ears. I tried to get up and then stayed where I was. It was the darkest dark I ever saw, complete blackness. I thought I was blind.

A torchlight beam cut through it, picked me out, and then moved on. "What was it?" I shouted. "What happened? Was it a quake?"

"Stop yelling," Konski's voice answered me casually. "That was no quake, it was some sort of explosion. Mr. Knowles-you all right?"

"I guess so." He gasped for breath. "What happened?"

"Dunno. Let's look around a bit." Konski stood up and poked his beam around the tunnel, whistling softly. His light was the sort that has to be pumped; it flickered.