Some of the chambers were half-way to the roof and were reached by steps cut in the solid rock. d.i.c.k decided on one of these not far from where they were standing. They crept cautiously from their hiding-place and stole along to the bottom of the cave. A moment later they were at the foot of the steps. These they hastily climbed, and soon found themselves in a fair-sized cave, fifteen or sixteen feet above the floor of the main cavern and commanding a good view of the entire area. It was dry and warm and formed an ideal post of observation, provided their presence remained undiscovered.

Suddenly a blaze of light struck their eyes. Some one had turned on the whole of the electric lamps which hung in cl.u.s.ters from the roof.

Peering cautiously out they saw, to their amazement, half a dozen men issue from different chambers near the floor of the cave. All wore big round spectacles of deep blue gla.s.s and were clothed in close-fitting garments of rubber, with heavy gauntletted gloves of the same material.

Apparently they could not see well, for the spectacles must have been almost impervious to ordinary light.

One of the men, fixing his spectacles on more firmly and, drawing his rubber overall more closely around him, approached the strange machine which stood on the table. The others proceeded to the points at which the wires from the machine reached the side of the cave. Here they took up some kind of tool which looked like a gigantic blowpipe and stood ready as if awaiting a signal.

A low whistle sounded from the man at the table, as he grasped a small wheel and gave it a quick turn.

An instant later an appalling blaze of light burst from the strange machine, and the cave was filled with a roar of sound, a terrible deep drone of such frightful intensity that the hidden watchers shuddered as if with actual physical agony. d.i.c.k felt the sweat start suddenly from his forehead and pour down his face. Anxiously he glanced towards Yvette. She lay with her face buried in her arms, her body trembling convulsively. Scott and Jules, their faces white as chalk, were gazing at the unearthly light which streamed from the whirling machine, shading their eyes with their hands to shelter them from its blinding radiance.

They could not look at it for more than a few seconds; it was like trying to gaze at the sun at midday.

Taking a letter from his pocket, d.i.c.k bored a tiny hole in it with his scarf pin. Through this hole he found he could see in comparative comfort. He signed to the others to do the same, and soon all four--for Yvette quickly recovered her self-possession--were eagerly watching the strange scene before them. Speech, in the deafening noise by which they were surrounded, was, of course, out of the question.

The man at the great table in the centre of the cavern evidently had a task of great difficulty to control the movements of the strange machine, which he seemed to do by means of a large wheel something like the steering wheel of a steamer. Long streamers of flame shot from it in all directions, and as its ma.s.s of wheels revolved at terrific speed it shook and trembled as if it would actually leap from the table.

In the meantime the men at the rock face were hard at work with big blowpipes, from the muzzles of which shot streams of fire of such intensity that the solid rock seemed to melt away like b.u.t.ter. The molten matter was led by ducts in the ground through a grid of some metal, evidently highly refractory to heat, for it appeared to do no more than glow white-hot even in the terrific temperature of the melted rock. After pa.s.sing through this grid the molten matter was led to the bed of the stream, from which the water had in some manner been cut off, and flowed out the way d.i.c.k and his companions had entered.

What was the object of the work?

d.i.c.k could not guess, but every now and again one of the men would walk to the grid and with a long implement shaped like a hoe would sc.r.a.pe off something adhering to the bars, which he deposited in a big tank of water. d.i.c.k determined that, sooner or later, he would obtain a specimen.

But in the meantime their position was decidedly precarious. If they were observed there was no possible way of escape, for the tunnel by which they had entered was barred by the stream of molten matter. They could only lie still and hope that no one would enter the gallery in which they lay concealed.

After two hours of work, the man at the table stopped the machine, and all the men straightened out for a rest. Evidently they were very much exhausted. The lights were extinguished, except for the single one which was burning when they entered, and the men returned to their quarters, evidently almost falling with weariness. d.i.c.k came to the conclusion that they could only carry on the work on which they were engaged for a short time and that after that sleep and rest were imperative. The flow of molten metal had stopped and the water was again allowed to flow along its ordinary channel, from whence it sent up huge clouds of dense steam.

This gave d.i.c.k his chance.

Sending the others to the mouth of the exit, he cautiously crept towards the tank in which were deposited the sc.r.a.pings from the grid which filtered the molten rock. He reached it safely, and plunging in his arm up to the shoulder, abstracted a couple of handfuls of what seemed like heavy shot. These he placed at once in his pocket.

He was about to return to the others when his attention was caught by the queer platform at the one end of the cave. Looking at this carefully he found that it was really a huge lift, and at once the mysterious disappearance of the sailor and the mules was explained. It was evident that the top of the lift was really the thin covering of rock which had sounded hollow when tapped and that this had been so cut that when the lift forced it into position only traces of ragged crevices were left on the surface. d.i.c.k could not but admire the ingenuity with which this approach to the subterranean retreat had been devised.

Presently he heard a heavy knocking above his head and, guessing the cause, shrank back for shelter into the mouth of a small cave adjoining.

A moment later a man emerged from one of the other chambers and approached the lift. d.i.c.k was curious to see how it worked. There was, as he could see, a small electric motor fitted to it, but where could the necessary power come from?

The new-comer carried in his hand a tiny machine which was in every respect a duplicate in miniature of the big one on the central table.

But it was so small that the man carried it easily in one hand. From it ran a pair of electric cables which the man proceeded to connect with the terminals of the motor.

Placing the machine on the ground he gave the wheel a sharp turn.

Immediately the tiny machine began to revolve, throwing out flashes and flames exactly like the larger one but on a miniature scale.

Clearly, however, there was considerable power in it, for the lift at once commenced to descend. On it stood a man whom d.i.c.k instantly recognised as Gronvold. And he was accompanied by the sailor whom d.i.c.k had left safely tied up in their camp. Evidently Gronvold had found and released him.

Their position was now indeed one of terrible gravity.

As soon as the lift reached the bottom the two men stepped off and the lift reascended, moving upward with an ease which showed the tremendous power developed by the tiny machine. Here, indeed, was something of which d.i.c.k had had no previous experience.

The three men crossed the cave to the shelter occupied by the man who worked the big machine, who was evidently the captain, and d.i.c.k knew there was no time to be lost.

Directly the men entered the shelter, d.i.c.k dashed across the cave to join the others, s.n.a.t.c.hing out his revolver as he ran.

He had nearly reached them, when a whistle blew and instantly half a dozen men rushed from different caves. They were discovered!

"Take care of Yvette, Jules!" d.i.c.k yelled as, with Scott at his side, he faced round to the men who were rushing at them from three sides.

Instantly Yvette and Jules plunged into the tunnel. d.i.c.k and Scott backed after them with drawn revolvers threatening the men in the cave.

For a moment the leaders hesitated; apparently they were not aimed.

Then Gronvold rushed to the front, followed by the captain, both carrying curious weapons which looked like heavy pistols.

All four men fired simultaneously. d.i.c.k saw the captain drop, evidently shot dead, and heard a bullet whiz past him and strike the rock behind.

A burst of flame singed his hair, and he felt the hot breath of it on his face.

Then Gronvold fired at Scott. The effect as the bullet struck him was strange and awful. His body actually disappeared in a ma.s.s of flame under the impact of some projectile of unimaginable power and energy.

At the same instant d.i.c.k slipped on a projecting bit of rock and fell heavily on his head. As he lost consciousness he heard the crack of a revolver behind him. Yvette and Jules, hearing the shots, had returned in the nick of time. Jules s.n.a.t.c.hed up d.i.c.k and carried him down the tunnel, while Yvette very coolly shot down Gronvold just as he was reloading his terrible weapon.

When d.i.c.k recovered his senses he found himself lying on the ground at the entrance to the tunnel, his head pillowed on Yvette's arm as she tried to pour some brandy between his lips. He could feel the sobs which shook her, and even felt a tear on his face. Jules stood on guard at the entrance to the tunnel, his revolver ready for instant action in case of pursuit.

As d.i.c.k opened his eyes, Yvette gave a gasp of relief.

"Oh, dearest, I thought you were dead!" she sobbed and burst into tears.

A moment later she turned away blushing scarlet. She had betrayed her secret at last. And even in his confused state d.i.c.k felt a thrill of triumphant joy.

His head spinning he staggered to his feet. But he would have fallen if Yvette had not caught him.

"Sit down, d.i.c.k," she said peremptorily. "Jules can look after this place."

d.i.c.k obeyed, perforce; he was so sick and giddy that he could have done nothing even if the expected attack had come.

But it never came. Suddenly as they stood there, tense and waiting, a terrific convulsion shook the earth. With a terrible roar the great cavern collapsed and a vast burst of smoke and flame vomited to the sky, and a deep crater was left by the subsidence. Sick and dizzy, with showers of stones falling all around them, they stood aghast while explosion after explosion rent the air, rendering the crater deeper. It was some minutes before quiet reigned again and, white and shaken, after their nerve-racking experience, they were able to collect their shaken faculties and make an examination of the scene.

The hill beneath which the cavern was located had practically disappeared; in its place was left nothing but a heap of torn and tumbled earth and rock. Its dreadful secret was safe, for the cave and its contents, and the men who had wielded such t.i.tanic forces, were buried deep under tens of thousands of tons of debris.

Perhaps it was as well, d.i.c.k thought. There are some forms of knowledge which mortals ought not to possess; there are some powers which they are not fit to handle.

Whatever secret Gronvold had discovered, it rested with him for ever on the very scene of his ill-omened labours. What had gone wrong in the depths of the cavern they could not even imagine, but it was evident that the mysterious force which Gronvold had called into existence, whatever it was, had destroyed him and his companions. And it was almost by a miracle that d.i.c.k, Yvette, and Jules had escaped.

Slowly and painfully they made their way back to their camp, and for the first time d.i.c.k became conscious of the great weight of the double handful of shot which he had taken from the tank. He drew some of it out and examined it by the light of the fire. As he did so he gave a cry of surprise. For the "shot" was nothing more or less than tiny nuggets of virgin gold.

Here was an addition to the mystery. As d.i.c.k knew perfectly well, there was not an atom of gold-bearing rock within hundreds of miles of where they stood.

It was evident that one of the secrets of Gronvold's invention was that it gave him the power of actually bringing about the trans.m.u.tation of substances. There was some element in the rock which was susceptible of being changed into gold by a process at which they could not even guess.

But if this were so, Gronvold had indeed, as they suspected, been able to solve the problem of loosing the incredible force contained in the atom. His discovery was, as d.i.c.k at once realised, on the lines of the latest development of scientific thought.

d.i.c.k was to see the problem solved in later years by more reputable investigators.

But he could never forget his strange encounter with the wonderful but misguided genius whose career had been so terribly brought to an end by the dread power he had himself evoked.