Strangers

the tunnel entrance, and the outer door hissed shut behind him and Lieutenant Horner.
        Immediately, a pair of video cameras, mounted on the ceiling at opposite ends of the chamber, clicked on. The cameras tracked the two men as they walked to the inner door.
        No human eyes were watching the colonel and lieutenant on any video display, for the system was operated entirely by VIGILANT, the security computer, as a precaution against the possibility that a traitor within Thunder Hill's own guard unit might open the facility to hostile forces. VIGILANT was not linked to the installation's main computer or to the outside world; therefore, it was invulnerable to saboteurs seeking to take control of it by means of a modern or other electronic tap.
        The guard at the perimeter fence had notified VIGILANT that Colonel Leland Falkirk and Lieutenant Thomas Homer would be arriving. Now, as they approached the inner door under the gazes of video cameras, the computer compared their appearance to stored holographic images of them, rapidly matching forty-two points of facial resemblance. It was impossible to deceive VIGILANT either with makeup or with a look-alike for an approved visitor. If Leland or Horner had been an imposter or unauthorized visitor, VIGILANT would have sounded an alarm, simultaneously filling the entrance tunnel with a sedative gas.
        The lock on the inner door had no keyboard; no code would open it. Instead, a one-foot-square panel of glass was set in the wall beside the door. Leland almost pressed his right hand palm-down against the panel, hesitated, then used his left, and the glass lit, and a faint humming arose. VIGILANT scanned his palmprint and fingerprints, comparing them to the prints in its files.
        Lieutenant Horner said, "Almost as hard to get in here as into heaven."
        "Harder," Leland said.
        The light behind the milky glass winked out, and Leland took his hand away. The inner door opened.
        They stepped into a huge natural tunnel that had been improved by human hands. The domed rock overhead was lost in darkness because the lighting fixtures were suspended from black metal scaffolding, creating the illusion of a ceiling perhaps twenty or thirty feet below the true ceiling. The tunnel was sixty feet across and led into the mountain about a hundred and twenty yards. In some places the rock walls had natural contours, but in other places they carried the imprints of dynamite blasts and jackhammers and other tools that had been used to widen the narrow portions of the passageway. Incoming trucks could drive along the concrete floor to unloading bays beside immense cargo elevators that went down into deeper regions of the facility.
        A guard sat at a table beyond the door by which Leland and Horner entered. Considering the remoteness of Thunder Hill, the extent of sophisticated defenses, and the thoroughness with which VIGILANT examined all visitors, a lone sentry seemed superfluous to Leland.
        Evidently, the sentry was of that same opinion, for he was not prepared for trouble. His revolver was holstered. He was eating a candy bar. Reluctantly, he looked up from an old novel by Jack Finney.
        He wore a coat because the open areas of the Depository were never heated; only the enclosed living quarters and work areas were kept warm. An enormous power supply was provided by a mini hydroelectric plant that harnessed an underground river, plus backup diesel generators, but there was not enough to warm the mammoth caverns. The subterranean temperature was a stable fifty-five degrees, quite bearable if one dressed for long work periods in the chilly air, as the guard had done.
        He saluted. "Colonel Falkirk, Lieutenant Horner, you're cleared to see Dr. Bennell. You know how to find him, of course."
        "Of course," Falkirk said.
        Ten feet to the left, the burnished steel surface of the giant blast doors glimmered softly in the fluorescent light, looking rather like the sheer face of a great glacier. Leland and Lieutenant Horner turned right, away from the big doors, and walked deeper into the mountain, toward the elevators.
        Thunder Hill Depository was equipped with hydraulic lifts of three sizes, the largest of which rivaled the enormous elevators on aircraft carriers. A carrier's lifts were used to bring planes from the ship's hold onto the flight deck, and Thunder Hill's also lowered and raised