“You are now a full mile beneath the surface of Mercury,” said Agvur, “and the part of the twilight zone in which your vessel lies is about five miles away and could readily be reached by an incline leading upward to a small exit in a natural cavern within sight of the ship. Doubtless some of the members of your party have seen the cavern and have assumed that it was a mere animal-den. When your vessel landed, we took care to block the exit with a few loose boulders and fragments of detritus, easily removable.

“As to rejoining your comrades—well, I fear that it will scarcely be practicable. You must be our guest—perhaps indefinitely.” There was a kind of brusqueness in his tone as he concluded:

“We do not want our existence known to terrestrial explorers. From what we have seen of your world, and your dealings with the peoples of Mars and Venus, whose territories you have begun to arrogate, we think it would be unwise to expose ourselves to human curiosity and rapacity. We are few in number, and we prefer to remain in peace—undisturbed.”

Before Howard could frame any sort of protest, there came a singular interruption. Loud and imperious, with clarion-like notes, a voice rang out in the empty air between Agvur and the earth-man. Howard was ungovernably startled; and the three Mercutians all seemed to stiffen with rapt attention. The voice went on for nearly a minute, speaking rapidly, with accents of arrogant command. Howard could make nothing of the words, whose very phonetic elements were strange and unfamiliar. But a chill ran through him at something which he sensed in the formidable voice—a something that told of relentless, implacable power.

The voice ended on a high, harsh note, and the listening Mercutians made a queer gesture with their heads and hands, as if to indicate submission to a superior will.

“Our temporal ruler and chief scientist, Ounavodo,” said Agvur, “has just spoken from his hall in the lower levels. After hours of deliberation, he has reached a decision regarding your fate. In a sense, I regret the decision, which seems a trifle harsh to me; but the mandates of the Shol, as we call our ancient ruler, are to be obeyed without question. I must ask you to follow me; and I shall explain as we go along. The order must be executed without delay.”

In perplexity not unmingled with consternation, Howard was led by Agvur to a sort of inclined hall or tunnel, on which the chamber opened. The tunnel was seemingly interminable, and was lit by brilliant and apparently sourceless light—the transmitted sun-rays of which the Mercutian had spoken. Like the chamber, it was lined with a pale metallic substance.

An odd machine, shaped like a small open boat, and mounted on little wheels or castors, stood before the door, on the easy monotonous grade. Agvur stationed himself in its hollow prow, motioning Howard to follow. When the other Oumnis had placed themselves behind Howard, Agvur pulled a sort of curving lever, and the machine began to glide rapidly, in perfect silence, down the interminable hall.

“This tunnel,” said Agvur, “runs upward to the exit near your vessel; and it leads down to the heart of our underworld realms. If the worst happens—as I fear it may—you will see only the antechambers of our labyrinth of caverns, in which we have dwelt, immune to disease and old age, for so many centuries. I am sorry; for I had hoped to take you to my own laboratories, in the nether levels. There you might have served me…in certain biologic tests.

“Ounavodo,” he went on, in calm explanatory tones, “has ordered the fusing and casting of a certain quantity of the mouffa-alloy, to be used in the making of new garments. This alloy, invented aeons ago by our metallurgists, is a compound of no less than six elements, and is made in two grades, one for the lining of our caverns, and the other exclusively for raiment.

“Both, for their perfection, require a seventh ingredient—a small admixture of living, protoplasmic matter, added to the molten metal in the furnace. Only thus—for a reason that is still mysterious to our savants—can the mouffa acquire its full power of insulation against the deadly solar rays.

“The mouffa used in comparatively heavy sheets for cavern-lining needs only the substance of inferior life-forms, such as the grokos, the half-animal savages of the twilight zone, and various creatures which we catch or breed in our underworld tunnels. But the higher grade of mouffa, employed in light, flexible sheets for suiting, requires the protoplasm of superior life.

“Regretfully, at long intervals, we have been compelled to sacrifice one of our own scanty number in the making of new metal to replace that which has become outworn. Whenever possible, we select those who in some manner have offended against our laws; but such infringements are rare, and commonly the victim has been chosen by a sort of divination.

“After studying you closely in his televisic mirror, Ounavodo has decided that you are sufficiently high in the evolutionary scale to provide the protoplasmic element in the next lot of mouffa. At least, he thinks that the test is worth trying, in the interests of science.”

“However, in order that you should not feel that you are being discriminated against or treated unjustly, you will merely take your chance of being chosen from among many others. The method of selection will be revealed to you in due time.”

While Agvur was speaking, the vehicle had sped swiftly down the endless incline, passing several other barge-shaped cars driven by the white, naked Immortals, whose serpentine locks flowed behind them on the air. Occasionally there were openings in the tunnel wall, leading no doubt to side-caverns; and after a mile or two, they came to a triple branching, where caverns ran upward at reverse angles from the main passage. Horrified and shaken as he was by Agvur’s disclosure, Howard took careful note of the route they were following.

He made no reply to the Mercutian. He felt his helplessness in the hands of an alien, extra-human race, equipped, it would seem, with scientific knowledge and power to which humanity had not yet attained. Thinking with desperate quickness, he decided that it would be better to pretend resignation to the will of his captors. His hand stole instinctively to the pocket in which he had carried the little tonanite pistol with its twelve charges of deadly heat-producing explosive; and he was dismayed, though hardly surprised, to find that the weapon was gone.

His movement was noted by Agvur; and a strange sardonic smile flickered across the unhumanly intellectual face of the savant. In his desperation, Howard thought of leaping from the car; but to do this would have meant death or serious injury at the high speed of their descent.

He became aware that the incline had ended in a large level cavern with numerous side-openings where multitudes of Oumnis were passing in and out. Here they left the boat-like vehicle; and Howard was led by Agvur through one of the side-exits, into another vast chamber, where perhaps fifty of the white people were standing in silent, semi-circular rows.

These beings were all fronting toward the opposite wall; but many of them turned to watch the earth-man with expressions of unreadable curiosity or disdain, as Agvur drew him forward to the first of the waiting ranks and motioned him to take his place at the end.

Now, for the first time, Howard saw the singular object which the Oumnis were facing. Apparently it was some sort of rootless plant-growth, with a swollen, yellowish bole or body like that of a barrel-cactus. From this body, tall as a man, leafless branches of vivid arsenic green, fringed with a white hispidity, trailed in limp, sinuous masses on the cavern-floor.

Agvur spoke in a piercing whisper: “The plant is called the Roccalim, and we employ it to choose, from a given quota, the person who shall be cast into the furnace of molten mouffa. You will perceive that, including yourself, there are about fifty candidates for this honor—all of whom, for one reason or another, in varying degrees, have incurred the displeasure of Ounavodo, or have given rise to doubt regarding their social usefulness. One by one, you are to walk about the Roccalim in a complete circle, approaching well within reach of the sensitive, mobile branches; and the plant will indicate the destined victim by touching him with the tips of these branches.”