"Okay, okay," I interrupted, "so the place is safe... 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.

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...

Knowles sighed. "...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."

"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.