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 Dick’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 massive 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 balls, it was a complicated mass of polished wheels, of some metal which Dick could not identify, and which gleamed with a strange radiance in the light of the huge electric lamp overhead. From the machine a bewildering mass of wires led to a series of points at the face of the rock.

So much Dick 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. Dick 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?

Some of the chambers were half-way to the roof and were reached by steps cut in the solid rock. Dick 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 clusters 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 glass 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. Dick 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.