The Cosmic Computer by H. Beam Piper

They sent a snooper in first; it picked up faint radiation leakage from inactive power units of overhead lights, and nothing else. The tunnel stretched ahead of it, empty, and dark beyond its infrared vision. After it had gone a mile without triggering anything, the jeep followed, Anse Dawes piloting and Conn at the snooper controls watching what it transmitted back. The two lorries followed, loaded with men and equipment, and another jeep brought up the rear. They had cut screen-and-radio communication with the outside; they weren't even using inter-vehicle communication.

At length, the snooper emerged into a big cavern, swinging slowly to scan it. The walls and ceiling were rough and irregular; it was natural instead of excavated. Only the floor had been leveled smooth. There were a lot of things in it, machinery and vehicles, all battered and in poor condition, dusty and cobwebbed: the spaceport junkheap. A passage, still large enough for one of the gunboats, led deeper into the mountain toward the crater. They sent the snooper in and, after a while, followed.