The basic location had suggested itself naturally enough. Gantt had briefly considered a shipboard experiment in the ocean west of San Diego, but he concluded that the delicate measurements he hoped to make would be virtually impossible with the present equipment on board a pitching ship. Even on this solid land where he now stood, the natural tremors of the Earth could mask any small effect, and he did not really know what effect to anticipate.
He mentally surveyed the layout. Arrayed over several miles of barren rolling desert were a series of seismometers to measure the ordinary activity of the Earth, and the special seismic waves that were due to be superposed. There were also the special instruments designed to detect any accelerations that might occur if a significant gravitational pull, in addition to that of the Earth, were to occur. All these instruments were connected to a small but powerful computer energized by the portable generator the noise of which disturbed the otherwise quiet early afternoon. This computer would provide an instant analysis of the data. It not only recorded the strength of the signals but, using information from instruments spaced at a distance, it could also triangulate and determine the direction and distance to the source of the waves or gravitational acceleration.
All was now in readiness. Gantt felt a small chill despite the heat. In a little over an hour the seismic waves should broach the surface about two hundred miles away in eastern Arizona, registering on the seismometers but perhaps only marginally on the accelerometers even with Runyan’s most extreme estimates. Eighty and a half minutes later the source of the waves would again approach the surface but a thousand miles to the west, over seven hundred miles off the Pacific coast. Since the incommensurate period of rotation of the Earth made the surfacings appear to shift one hundred ninety miles every twenty-four hours, tomorrow at nearly the same time the waves should impinge on the surface very close to their present location.
Gantt turned his back on the encampment and looked out across the shallow hills. He had great difficulty accepting the picture proposed by Runyan, and yet he could not resist a morbid temptation to imagine what was proceeding if the hypothesis were correct. A small speeding object was now plunging down through the deepest basalt layers of the Earth’s crust. In fifteen minutes it would enter the molten core, picking up speed as it went. Sensing the change in gravitational pull as it passed the Earth’s center, it would begin to slow as it shot back toward the surface, where it would peak with majestic slowness before crashing back into the dirt and rock.
Gantt shook his head and strode back to the main tent of the encampment. The interior of the tent was a little cooler because of the air conditioner installed to service the computer, but it was still stifling. Gantt became too engrossed to notice.
At five minutes before the appointed time, he focused his attention on the needles of the seismometers. They jiggled steadily, but with nearly constant amplitude, tuned to the basic constant sounds of the Earth. In a couple of minutes he saw the effect he was looking for. The swings of the needles on all three seismometers began to slowly grow in amplitude. Danielson’s seismic waves were real enough all right. The question was what caused them. Even to Gantt’s trained eye the signals on the three instruments looked identical. Only the computer could distinguish the minute differences due to the slightly different distances of the instruments from the source of the waves. Gantt turned to the computer, typing rapidly on the keyboard and then scrutinizing the screen in front of him as the printer to one side began to roll out the same data on a chain of paper sheets. The distance was about one hundred ninety miles, a little closer than their best guess, but within the expected errors. Gantt’s gaze then swung to take in the readings from the accelerometers that might detect some variation in gravitational force. He thought he could make out the briefest fluctuation, but could not be sure. Again he keyed the computer and found his impression confirmed. There might be an effect, but it was only marginally above the noise level. A more sophisticated analysis that could only be done with time and a bigger computer might dig something out, but for now there was no firm conclusion to be reached. Still, he mused, an effect of the size Runyan predicted could not be ruled out. If the minute fluctuation were real, then something massive had just surfaced two hundred miles away, and in three quarters of an hour it would do so again on the far side of the Earth.
Gantt stripped the printed computer output off the machine and examined it more carefully. He swore quietly as sweat dripped off his brow onto the paper, obscuring a few numbers. He stopped to wipe his forehead and neck and then returned his attention to the rows of numbers. The seismic waves stopped several miles below the surface. After a minute or so, the source of the waves began again, moving nearly vertically down into the Earth. Gantt felt a nervous tightening across his abdomen. An ordinary seismic wave could be reflected, but it did not wait a minute while making up its mind. Such a delay might occur if the source of the waves moved up into light surface layers that were not conducive to the production of waves and then fell back again. Runyan’s hole could do that. Deep in thought, Gantt sat for some minutes striving for an explanation in terms of the normal behavior of the Earth as he knew it. Nothing occurred to him, but he told himself that Runyan need not be right on that basis, perhaps it was just his own lack of imagination or lack of sufficient information. The mysterious interior of the Earth had surprised him more than once and might be doing so again. Taking solace from that thought, he proceeded to a close study of the data acquired during the event.
Wednesday morning Pat Danielson clambered down from the rear seat of the jet-black F-16 that was rigged for tactical reconnaissance. She was aided by the pilot and a ground technician. Her legs were a little unsteady from the excitement of the Mach 2 flight from Washington—over two thousand miles to the Yuma Air Station in an hour and a half. She followed a young marine to a waiting helicopter and stood there while he went into a nearby utilitarian terminal building. He reemerged in a moment followed by Alex Runyan. Runyan was halfway across the tarmac when he looked up and saw her. The look of surprise and pleasure on his face was delicious to her.
“Pat!” He ran forward, grabbed her hand in both his and gave her a spontaneous peck on the cheek, oblivious to the watching servicemen. “What a delight. I didn’t expect to see you here.”
“After you pleaded with Bob Isaacs yesterday,” Danielson said gaily, “we decided to coordinate the trips, save a helicopter ride.”
“That’s great. When did you leave? It’s a long way.”
She laughed with obvious glee. “Crossing three time zones helps, but so does that,” she pointed toward the fighter. “We landed before we took off.”