With great care he got the four mules together, standing side by side.

He himself took up a position directly in front of them and almost touching the animals' heads.

A moment later man and mules sank together, apparently into the earth and disappeared!

They could hardly believe their eyes! Surely the man must have gone down the reverse slope of the hill. But they were confident that he had not moved.

They hurried to the spot. Not a sign of any living thing was to be seen! The mystery was profound.

While they stood gazing at one another in speechless amazement, the Mohawk, which they had not perceived above them, dropped vertically downwards and landed a few yards away. d.i.c.k sprang out.

"Did you see?" he gasped. "The man and mules went down into some sort of pit. But where was it?"

The flat top of the hill was broken into a series of narrow cracks; apparently the rock of which it was composed was of volcanic origin.

They examined it closely, but they could discover nothing which offered a solution of the mystery.

d.i.c.k described closely what he had seen from the sky. It agreed with what the others had observed. The man had got the mules together, and all had sunk slowly downward. d.i.c.k had seen the black mouth of the pit for a few moments and a blaze of light. Then the pit had disappeared, and the ground resumed its normal appearance.

"We shall have to camp here to-night," said d.i.c.k. "We must get to the bottom of this. We shall have to take turns to watch. In the meantime we had better have a look round."

Having closely examined the top of the hill, they turned to the deep gorge and descended to the bottom. The stream, they found, issued from the hill itself, flowing out from a low tunnel high enough to admit the pa.s.sage of a man. From it also issued a cloud of mist which spread over the bottom of the little valley in a thick blanket which completely concealed the surface of the ground from anyone at the top of the hill.

But still more remarkable was that the bed of the little stream was deeply covered with what appeared to be recently melted lava. In many places it was still hot, and the water, they found, was nearly boiling.

The first traces of this were found at the mouth of the tunnel from which the stream emerged, and for hundreds of yards the molten rock could be traced, as though it had poured from the tunnel and flowed down the bed of the brook.

Wood and water were available in abundance, and soon they had pitched their camp, near enough to the top of the mysterious hill to enable them to watch it closely and yet well concealed so that if the man reappeared they would have no difficulty in escaping observation.

The first watch fell to Yvette, and with a revolver ready for instant use, she prepared to spend a couple of lonely hours on the edge of the hill. The camp was but a quarter of a mile away so that a shot would bring her speedy help at any time.

A brilliant moon lit up the country for miles.

There was no trace of any living thing. Everything was still and silent.

Yvette had been on watch about an hour when she became aware that the air was full of a dull murmur of sound. She listened intently. There was no mistake about it. A dull throbbing noise was distinctly discernible.

She walked round the flat top of the hill, looking keenly in every direction and trying to locate the position from which the mysterious sound was coming. But it was in vain.

Glancing into the gorge, she saw a strange and terrible phenomenon. The course of the little brook was traced in a dull fiery glow. Clouds of steam were rising thickly into the night air; she could plainly hear the sharp hiss of water on something hot.

She ran swiftly down the hill. At the bottom she paused on the edge of the stream. The water had disappeared and in its place ran a river of molten rock! Through her boots she felt the heat of the ground.

Returning to the top of the hill she waited for d.i.c.k, who was now almost due to relieve her. In a few moments he appeared and listened in amazement as she gasped out her story.

The dull, throbbing noise was still audible.

"Machinery," said d.i.c.k laconically, "but where?"

Suddenly he flung himself on his face, and pressed his ear close to the ground.

"Listen," he said.

Yvette followed his example. There could be no mistake; the mysterious sound was coming from the ground beneath their feet! The earth was full of m.u.f.fled thunder.

d.i.c.k took from his pocket a hammer and struck a sharp blow on the flat rock beneath their feet. It rang hollow! Unmistakably they were standing on the roof of a cavern.

Walking to the camp they roused the others and told them what they had seen and heard.

"We have got to catch that sailor if we wait here a month," said Scott.

"He must come out again some time. But how about food?"

"We have enough tinned stuff in the Mohawk for a week," said d.i.c.k, "so we shall be all right for a few days. In the meantime we must watch the place closely."

Next day pa.s.sed without incident until evening was drawing on. Then Yvette, who was watching the top of the hill while the others rested, at six o'clock gave a low whistle. She was lying on the ground keeping observation between a couple of rocks which hid her completely. In a moment the others had crawled to her side.

"Look!" she said.

On the top of the hill, three hundred yards away, stood the sailor and the four mules, clearly silhouetted against the evening glow. He had appeared suddenly, Yvette told them, just on the spot where he had disappeared on the previous day.

"We must get him," said d.i.c.k.

The man with the mules started to return along the way he had come.

They saw at once that the path he was taking would bring him close to them.

With the mules unloaded the man evidently had no intention of walking.

He mounted one of the animals and rode towards them at a fast trot.

He was within twenty yards when d.i.c.k aimed his revolver and fired. The mule the man was riding bolted, throwing its rider heavily. Before he could recover himself he was bound and helpless. The other three mules stampeded wildly and were soon out of sight.

Carried to the camp the man soon recovered. But he resolutely refused to say a word.

"Well," said d.i.c.k. "We must try to get into the cave. Perhaps the tunnel out of which the brook runs will lead us to it."

They were soon at the mouth of the strange tunnel. There was no sign of the molten matter of the previous night. The stream, thick with mud, flowed sluggishly, but the water was cool, and the ground, which the night before had been too hot to walk upon, was now not more than uncomfortably warm.

With d.i.c.k leading, Scott and Yvette next in order, and Jules bringing up the rear they entered the mouth of the tunnel. There was, they found, just room for them to pa.s.s, stooping low and walking knee deep in the little stream. They were, of course, in total darkness, for d.i.c.k was afraid to show a light for fear of betraying their presence.

For a hundred yards d.i.c.k groped his way onward. Then his outstretched hands struck something soft. It was a kind of curtain hung across the stream, thick and heavy.

Cautiously he slightly raised one corner and peered through. The sight that struck his eyes filled them with amazement.

They were at the entrance to an enormous chamber, a hundred and fifty yards across, dimly lighted by a single big electric lamp, the only one alight out of dozens which hung from the roof. The floor sloped steeply upwards at the far end where they could make out a kind of platform, reaching nearly to the roof and with steps leading downward into the great hall. All round the side were a series of openings, apparently small chambers cut into the solid rock. From one of these the stream they had followed seemed to issue, crossing the floor of the great cave in a narrow deep channel.

But what fascinated d.i.c.k's attention was a great table, apparently of iron, which occupied the centre of the cave. It was heavily constructed and seemed to be based on ma.s.sive legs which went down into the rock.

Upon it stood a strange machine unlike anything he had ever seen before and of the use of which he could not form the smallest idea. Surmounted by two huge governor b.a.l.l.s, it was a complicated ma.s.s of polished wheels, of some metal which d.i.c.k could not identify, and which gleamed with a strange radiance in the light of the huge electric lamp overhead.

From the machine a bewildering ma.s.s of wires led to a series of points at the face of the rock.

So much d.i.c.k could make out in the dim light. He was keenly anxious to learn more. But how was it to be done? No sign of any human being was to be seen, but he could not imagine that what lay before their eyes was the work of the solitary sailor who now lay bound in their camp.

At any rate they could not remain where they were. d.i.c.k decided to try to gain entrance to one of the wall chambers where they could shelter with a better chance of seeing what would happen in this underground home of mystery. But which should they choose?