"You know, this thing was not built by men," Ray observed.

"Not built by men? What do you mean?"

"Men would have put the steps closer together. Jim, I'm afraid we are up against something--well--that we aren't used to."

"If men didn't build this, what did?" I was astounded.

"Search me! This continent has been cut off from the rest of the world for geologic ages. Such life as has been found here is not common to the rest of the earth. It is not impossible that some form of life, isolated here, has developed intelligence and acquired the power to erect that cone of light--and to burn the wing off a metal airplane."

My thoughts whirled madly as we clambered down the shaft.

It must have taken us an hour to reach the bottom. I did not count the steps, but it must have been at least a thousand feet. The air grew rapidly warmer as we descended. We both took off most of our heavy fur garments, and left them hanging on the rungs.

I was rather nervous. I felt the nearness of an intelligent, hostile power. I had a great fear that the owners of those steps would use them to find us, and then crush us ruthlessly as they had brought down Meriden's plane.

The little square of white light below grew larger. Finally I saw Ray swing off and stand on his feet in a flood of white radiance below me.

The air was warm, moist, laden with a subtle unfamiliar fragrance that suggested growing things. Then I stood beside Ray.

We stood on the bare stone floor of a huge cavern. It must have been of volcanic origin. The walls glistened with the sparkling smoothness of volcanic gla.s.s. It was a huge s.p.a.ce. The black roof was a hundred feet high, or more; the cave was some hundreds of feet wide. And it sloped away from us into dim distance as though leading into huger cavities below.

The light that shone upon us came from an amazing thing--a fall of liquid fire. From the roof plunged a sheer torrent of white brilliantly luminous fluid, falling a hundred feet into a shimmering pool of moon-flame. Shining opalescent mists swirled about it, and the ceaseless roar of it filled the cave with sound. It seemed that a stream of the phosph.o.r.escent stuff ran off down the cave from the pool, to light the lower caverns.

"Very clever!" said Ray. "They make the stuff up there at the cone and run it in here to see by."

"This warm air feels mighty good," I remarked, pulling off another garment.

Ray sniffed the air. "A curious odor. Smells like something growing.

Where anything is growing there ought to be something to eat. Let's see what we can find."

Only black obsidian covered the floor about us. Cautiously we skirted the overflowing pool of white fire, and followed down the stream of it that flowed toward the inner cavern. We had gone but a few hundred yards when suddenly Ray stopped me with a hand on my arm.

"Lie flat!" he hissed. "Quick!"

He dived behind a huge ma.s.s of fire-born granite. I flung myself down beside him.

"Something is coming up the trail by the shining river. And it isn't a man! It's between us and the light; we should be able to see it."

Soon I heard a curious sc.r.a.ping sound, and a little tinkle of metal. I caught a whiff of a powerful odor--a strange, fishy odor--so strong that it almost knocked me down.

The thing that made the sc.r.a.ping and the tinkle and the smell came into view. The sight of it sickened me with horror.

It was far larger than a man; its body was heavy as a horse's, but nearer the ground. In form it suggested a huge crab, though it was not very much like any crustacean I had ever seen. It was mostly red in color, and covered with a huge scarlet sh.e.l.l. It had five pairs of limbs. The two forward pairs had pinchers, seemingly used as hands; it sc.r.a.ped along on the other three pairs. Yard-long antennae, slender and luminously green, wavered above a grotesque head. The many facets of compound eyes stood on the end of foot-long stalks.

The amazing crab-thing wore a metal harness. Bands of silvery aluminum were fastened about its sh.e.l.l, with little cases of white metal dangling to them. In one of its uplifted claws it carried what seemed to be an aluminum bar, two feet long and an inch thick.

It sc.r.a.ped lumberingly past, between us and the racing stream of white fire. It pa.s.sed less than a dozen feet from us. The curious fishy smell of it was overpowering, disgusting.

Sweat of horror chilled my limbs. The monster emanated power, sinister, malevolent power, power intelligent, alien and hostile to man.

I trembled with the fear that it would see us, but it scrambled grotesquely on. When it was twenty yards past, Ray picked up a block of black lava that lay beneath his hand and hurled it silently and swiftly. It crashed splinteringly on the rocks far beyond the creature, on the other side of the stream of light.

In fascination I watched the monster as it paused as if astonished.

The glittering compound eyes twisted about on their stalks, and the long shining green tentacles wavered questioningly. Then the k.n.o.bbed limbs snapped the white metal tube to a level position. A metallic click came from it.

And a ray of red light, vivid and intense, burst from the tube. It flashed across the river of fire. With a dull, thudding burst it struck the rocks where the stone had fallen. It must have been a ray of concentrated heat. Rocks beneath it flashed into sudden incandescence, splintered and cracked, flowed in molten streams.

In a moment the intensely brilliant ruby ray flashed off. The rocks in the circle where it had struck faded to a dull red and then to blackness, still cracking and crumbling.

To my intense relief, the monstrous crab lumbered on.

"That," Ray whispered, "is what got Major Meriden's airplane wing."

When we could hear its sc.r.a.ping progress no longer, we climbed up from behind our boulder and continued cautiously down the cavern, beside the rushing luminous river. In half a mile we came to a bend. Rounding it, we gazed upon a remarkable sight.

We looked into a huge cavity in the heart of the earth. A vast underground plain lay before us, with the black lava of the roof arching above it. It must have been miles across, though we had no way to measure it, and it stretched down into dim hazy distance. Its level was hundreds of feet below us.

At our feet the glistening river of fire plunged down again in a magnificent flaming fall. Below, its luminous liquid was spread out in rivers and lakes and ca.n.a.ls, over all the vast plain. The channels ran through an amazing jungle. It was a forest of fungus, of mushroom things with great fleshy stalks and spreading circular tops. But they were not the sickly white and yellow of ordinary mushrooms, but were of brilliant colors, bright green, flaming scarlet, gold and purple-blue. Huge brilliant yellow stalks, fringed with crimson and black, lifted mauve tops thirty feet or more. It was a veritable forest of flame-bright fungus.

In the center of this weirdly forested subterranean plain was a great lake, filled, not with the flaming liquid, but with dark crystal water. And on the bottom of that lake, clearly visible from the elevation upon which we stood, was a city!

A city below the water! The buildings were upright cylinders in groups of two or three, of dozens, even of hundreds. For miles, the bottom of the great lake was covered with them. They were all of crystal, azure-blue, brilliant as cylinders turned from immense sapphires. They were vividly visible beneath the transparent water. Not one of them broke the surface.

Through the clear black water we saw moving hundreds, thousands of the giant crabs. The crawled over the hard, pebbled bottom of the lake, or swam between the crystal cylinders of the city. They were huge as the one we had seen, with red sh.e.l.ls, great ominous looking stalked eyes, luminous green tentacular antennae and k.n.o.bbed claws on forelimbs.

"Looks as if we've run on something to write home about," Ray muttered in amazement.

"A whole city of them! A whole world! No wonder they could build that cone-mountain for a lighting plant!"

"When they got to knocking down airplanes with that heat-ray," he speculated, "they were probably surprised to find that other animals had developed intelligence."

"Do you suppose those mushroom things are good to eat?"

"We can try and see--if the crabs don't get us first with a heat-ray.

I'm hungry enough to try anything!"

Again we cautiously advanced. The river of light fell over a sheer precipice, but we found a metal ladder spiked to the rock, with rungs as inconveniently far apart as those in the shaft. It was five hundred feet, I suppose, to the bottom; it took us many minutes to descend.

At last we stepped off in a little rocky clearing. The forest of brilliant mushrooms rose about us, great fleshy stalks of gold and graceful fringes of black and scarlet about them, with flattened heads of purple.

We started eagerly across toward the fungoid forest. I had visions of tearing off great pieces of soft, golden flesh and filling my aching stomach with it.