by Hal Clement
Had the agent cared about concealment, of course, he could have dug a little deeper—but all he wanted was good contact with bedrock. There was much mapping to do, and the matter of local life would have to wait until it was done.
II
The radar beams had stopped—or had, at least, ceased to reach the Conservation Agent—before he had gone underground. The point where he had landed was not in line-of-sight range of any of their stations. Needless to say, however, their operators had not forgotten him.
The agent was not considering possible radar operators. In fact, he would not have considered them even if radar had been, to him, something produced by a machine. He was far too busy listening.
If a human being puts his ear close against a wall, or a door-jamb in a fairly large building he will pick up a remarkable variety of sounds. He will hear doors closing, windows rattling, and assorted creaks and thuds whose origin is frequently difficult or impossible to determine. The one thing he will not hear is silence.
The crust of a planet is much the same on a vastly greater scale. It is always full of vibrations, ranging from gigantic temblors—as square miles of solid rock slip against similar areas on the two sides of a fault plane—to ghostly echoes of sound and the faint act of thermal oscillations as the sun’s heat shifts from one side of a mountain to the other, and the rocks expand and contract to adjust to the new temperature.
These waves travel, radiating from their point of origin, being refracted and reflected as they enter regions of differing density or elasticity, losing energy as they go by heating infinitesimally the rock through which they pass. They may die out entirely in random motion—heat—while still inside the body of the planet. Or a good, healthy wave-train may get all the way to the other side.
If it does so on Earth, it takes about twenty minutes. Then a fair proportion of it bounces from the low-density zone that is the bottom of the atmosphere, or the top of the lithosphere, whichever you prefer, and starts back again.
And every variation in density, or crystal structure, or elasticity, or chemical composition, has some effect on the way such waves travel. They may speed up or slow down. Transverse waves, or the transverse components of complex waves, may damp out—have you ever tried to skip rope with a stream of water?—and the compressional waves alone go through. Transverse waves, polarized in one direction, may be refracted through an interface, where the same sort of wave striking the same interface—at the same angle but polarized differently—may be reflected from it.
The important thing is that constantly varying conditions affect the waves. And that means that the waves carry information.
It is confused, of course. Temblors come from all directions, from all distances, due to many different causes, and through all sorts of rock. Interpreting them is not just a matter of sitting down to listen. One might as well tune in a dozen different radios to as many different musical programs, while sitting in the middle of a battlefield with a thunderstorm going on, and try to decide how many flutes were being used in one of the orchestras. The information is there, but selectivity and analysis are needed.
The agent was equipped for such selectivity, such analysis. His sensitive gear could detect any motion of the rock, down to thermal oscillation of the ions, at frequencies ranging from the highest a silicate group could maintain, to the lowest harmonic of a planet the size of Jupiter.
If his instruments proved inadequate, he could listen himself. But since just listening would involve the projection of a portion of his own body through the hull and bringing it into contact with the rock, the act would put a crippling strain on his stonelike flesh, and would consume several millennia of time. He did not plan to take this alternative. Machines were built to be used. Why not use them?
His own senses reacted at electronic speed—were, in fact, electronic in nature, as were his thought patterns. The process of receiving a group of impulses, and of solving the multiple-parameter equations necessary to deduce all the facts as to their origin and transmission, called for just such a fast-acting computer as his mind, though even he took some time about it.
This, primarily, was because he was careful. A temblor originating nearby would naturally have fewer unknowns worked into its waveform by the time it reached him. Therefore, it represented a simpler problem. Also, when solved, that problem provided quantities which could be fitted directly into the equations since their wave-trains must have come through the same rocks as they approached him.
His picture of the lithosphere around him grew gradually, therefore, and by concentric shells. He saw the layers of different sorts of rock and, far more important, the stresses playing on each layer—stresses that sometimes damped out to zero in the endless, tiny twitchings of the planet’s crust and that sometimes built up until the strength of the rocks, and the vastly greater weight of overlying materials could no longer resist them, and something gave.
He sensed the change, as trapped energy built up the temperature in a confined volume, until the rock could no longer be called solid, even though the pressure kept it from being anything that could be called liquid. He saw the magma pockets formed in this way migrate, up, down and across in the crust, like monstrous jellyfish in an incredibly viscous sea.
He saw certain points on the planet where they had reached so nearly to the surface that the weight above could no longer restrain the pressure of their dissolved gases. An explosive volcanic eruption is quite a sight, even from underneath.
His senses, through the vessel’s instruments, probed down toward the core of the world, where magma pockets were more frequent. In such pockets, held in solutions which might some day carry them to the upper crust, they would be accessible—the copper and silver and molybdenum, and other metals his people needed. They would lay diffused through the material of the planet.
Those were the things that interested him. He needed to know the forces at work down there—not in general, as a climatologist knows why Arizona is dry, but in sufficient detail to be able to predict when and where these metals would reach the upper crust and form ore bodies. The fastest electronic computers man has yet built would be a long time working out such problems, given the data. The agent was certainly no faster, and was less infallible.
He knew this to be so, and, therefore, spent much of his time checking and rechecking each step of the work. The task took all his attention, and, for the time being, he was totally indifferent to impulses originating near the surface—much less to a number of feeble ones which originated above the surface.
There was something a good deal more interesting than human reactions to claim the attention of the Conservation operative. He had, of course, confirmed long since his original impression that the ore beds of the planet had been looted. His principal job now was to decide how long the normal diastrophic and other geological processes would require to replace them.
On a purely general basis, replacement should take tens of millions of years for a planet of Earth’s size and constitution. Magma pockets would have to work their way up from the metal-rich depths to the outer crust. Then they would have to come into contact with materials which would dissolve or precipitate, as might be the case, the particular metals he sought.
The geological processes which depended so heavily on water or ammonia, in the liquid state, and concentrated the metallic compounds into ore deposits, could occur only near the surface. Of course, a magma pocket, commencing five hundred miles down, may not go upward. It may travel in any direction whatever, or not at all.
The density, the chemical composition, the melting point of the surrounding material, its ability to retain, in solution, the radioactives which may have been responsible for the pocket in the first place, were all vital factors. Equally vital was the question of whether its crystalline makeup is such as to absorb or release energy as increasing temperature reorganizes it—the proximity of one or more of the vast iron-pockets, whose coreward settling contributes its share of energy. All of
these things influence the path as well as the very existence of the pocket.
It would be relatively easy to predict, on a purely statistical basis, the number of ore-bodies to be formed in a given ten-million-year period. But the agent needed much more than that. When a freighter is dispatched to pick up metal at one specific point and deliver it to another, the schedule is apt to suffer if the ship has to wait a million years for its load. Interrupted schedules are not merely nuisances. In a civilization spread throughout the core of the galaxy, none of whose member worlds are self-sufficient, they can be catastrophic.
So the agent measured carefully, and, as he did so something a trifle queer began to appear. Impulses that did not quite fit into the orderly pattern he had deduced kept arriving—impulses of a nature he found, at first hard to believe.
Then he remembered that the poachers had been her for quite a while before his own arrival, and an explanation lay before him. The impulses were of the sort that his own hull must have broadcast, while he was digging his present refuge. There could be only one thing which the poachers would logically have left behind them. They could have left evidence of their digging.
They had shown, he decided, a rather unusual amount of foresight for their kind, coupled with a ruthlessness which made the agent wonder whether they had even felt the radar beams that had greeted his own arrival. What the poachers had done was not a thing to do to an inhabited planet.
The out-of-place impulses were from mole robots, slowly burrowing their way into the world’s heart. Each one, as the agent patiently computed its position, course, and speed, was headed for a point where the release of a relatively minute amount of energy would swing delicately balanced forces in a particular direction. The direction was obvious enough. The poachers expected to be back for another load, and were stimulating Earth’s diastrophic forces to provide it.
This was a technique often used by legitimate metal-producers, but only on worlds that were uninhabited. Orogeny, even when stimulated in this fashion, may take half a million years to raise a section of landscape a few thousand feet. That still would not provide time to escape for a being who, without mechanical assistance, would take something like the same length of time to travel a few hundred.
From the agent’s point of view, the presence of such depth-charges meant that Earth was going to become, in a fairly short time, a writhing, buckling, seething surface of broken rock, molten lava and folding, crumpling, tilting rafts of silicate material on a fearfully disturbed sea of stress-fluid.
Such heartless behavior might prove unavoidable—since he wouldn’t be there at the time. But—what had produced those radar beams?
It revolted him that any planet with life should be treated in such a manner. Whether or not the life was currently intelligent was beside the point. Few mutations were needed to transform a life species, from something as unresponsive as the planet that had pawned them, into a species capable of understanding the internal mechanism of a star in detail, for any distinctions of that nature to carry weight. If those beams had originated from living bodies, something would have to be done about the moles.
The agent simply did not have the equipment to do a thing. He could fight his little ship. He could investigate and analyze. He could communicate all the way across the galaxy, if something like the ionized layers of a planet’s atmosphere did not interfere.
But he had no mole robots on his vessel, no weapons that would penetrate rock, or even atmosphere, for any great distance. He could not himself stand the temperatures at depths to which some of the poacher’s moles had already penetrated. Consequently, he could not follow them in his own ship, even if it were able to dig as rapidly as the robots. It was indeed a problem!
Sending for help was possible, but almost certainly useless. His patrol area was so far out near the galactic rim that any message would take several millennia to reach a point where it would do any good—and the ships which answered it would be at least three times as long in covering the distance as the radiation that summoned them.
By then most, if not all, of the robots would have reached their designated target points. They would have shut off the fields which held their shape against the pressure of the surrounding rock. Once that protection was gone, no material substance in the universe could keep the half-ton of fissionable isotopes forming their cargoes at subcritical separation. All that energy would come out, and the little that wasn’t heat to start with soon would be.
Of course, even such amounts of energy are small in comparison with the usual supplies of a planet’s crust. But once released in carefully calculated spots and at even more carefully calculated times they would do exactly what the poachers wanted. The Conservation agent, checking the placement of the moles, could find no fault with the computations of the poachers’ geophysicist. He was in his own way an operator of genius!
He could, of course, arrange for official freighters to be on hand when the action bore fruit, and would certainly do so, as a last resort. But he must first attack the question of whether or not life was being endangered. For the first time since the beginning of his analysis, the agent directed his attention to the surface layers of the world.
Then he almost stopped again, as a new theory struck him. This planet had free oxygen in its atmosphere. Would its life, if any, be near the surface? But his hesitation was only momentary. He recalled the radar beams which were his only reason for suspecting life. They could not possibly have passed through any significant amount of rock. While his senses swept the surrounding crust, in ever-widening circles, he pondered the question of just how a living creature could endure such an environment. Think hard now, concentrate!
There was one obvious possibility. It might be riding a machine designed to protect it, as he was himself—which would imply that life was not native to this world. If that were the case, locating the creature or creatures should be easy. However, in such circumstances it would have to be assumed that the population was very small, since furnishing machines for all of a large population was a manifest impossibility. It would be unwise too—even if such a thing were possible.
A more fantastic idea was that, while the life of this world might have a carbon composition like his own, its metallic parts were of more inert substances—perhaps of the platinum-group metals. The agent knew no reason why these should not serve as well as calcium, in a nervous system. He might have thought of aluminum, had he been familiar with its behavior in an oxygen-water environment.
Then, there was the notion that a ship of his own race might be down and crippled—the most fantastic of all.
No such ship would be this far out in the galaxy, and it is hard to imagine a mishap which would leave the operator alive and safe from the environment, while crippling his communication facilities to the point where nothing but crude whistles came through.
Furthermore, there had been too many points of origin for the beams that had touched him. It might prove a tough nut to crack.
In fact, it was simply impossible to decide whether one of these hypotheses, or something which had not yet occurred to him would prove closest to the truth. For the time being, there was nothing to do but search. Naturally, it did not take long for the rhythmic impulses originating only a few miles away to catch his attention.
They were seismic, of course, since he was doing all his listening through the rock—but it quickly became evident that they were originating at the very boundary between lithosphere and atmosphere. Almost as quickly, he realized that the sources were moving.
This latter fact complicated the analysis rather seriously. It took the agent some time to conclude that sets of more or less solid objects, apparently always in pairs, were striking the lithosphere from outside. Sometimes there were relatively long periods of regular, repeated thuds, as one or more of the pairs did its hammering and such periods were always accompanied by motion of the point at which the blows were occurring.
At other times, the hammering was irregula
r, both in frequency and energy, and usually, though not always, these sequences radiated from a relatively fixed broadcasting point. There seemed to be six basic units producing the impulses. Well, he was making progress, at any rate. Systematic thought could be a joy in itself!
Quite evidently, if this disturbance were caused by local life, that life must be civilized to the point where it could design and build machines. Furthermore, six machines, machines so close together, really did call for thought. It suggested something about the population density of the planet.
On the worlds the agent knew, scarcely one individual in a thousand manned a machine capable of moving him about. To equip the rest similarly would not only be the height of folly, it would be impossible, because enough material could never be obtained; still more because very few of them were temperamentally suited to physical activity. Even if this race had equipped, say, one in a hundred of its members, the finding of such a number congregated in one spot implied either a tremendous population density or—could it be that they were looking for him?
He had never stopped to think what a two-dimensional search would be like. But these machines, he was beginning to think, must be confined to surface travel—perhaps sub-surface as well—and their operators were assuming that he was on or near the surface of the lithosphere.
The agent cast his memory back over the paths these things had been following, and decided that they might indeed be explained on the assumption they were seeking something and had a very restricted range of sensory perception. He dwelt for an instant on the last assumption, finding it unpleasant.
The radar beams, then, must have been used to track him. He had felt no such impulses, since digging in, although a portion of his hull remained exposed. But his attention had been so completely taken up with his work that he might not have noticed. He began to listen more carefully for electromagnetic radiation, and heard it immediately. On the instant, any doubts that might have remained concerning the intelligence of this race were disposed of.