Part 14 (1/2)

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.