The entrance to the Caves is under the edge of the Cataract over there, and by waiting here they are sure to be on hand when you arrive. Only"—her brows puckered in a little frown—"I don't understand why they remain out there on the open rock after Helgers has picked a hiding-place for the ship."

"Helgers?"

"He is the leader of the gang, and he is the man who killed that poor old Martian aboard the Western Star for the map. Helgers learned about the treasure and the existence of the map through a convict who was with Lozzo in the prison. Helgers pretends to be an importer in Chicago—he actually owns a nice little business there—but in reality he is one of the biggest smugglers in the Universe."

"How do you come to be with him?"

"I was coming to that," she replied. "My parents live on Ganymede."

Penrun nodded. He was familiar with the fourth satellite of Jupiter and its fertile provinces.

"My father is an American, but my grandfather on my mother's side was a Medan nobleman. He was ruined by that notorious pirate, Captain Halkon, who descended with his ships on our city and carried off everything of value, including the vast amount of scrip credits owned by the state which were entrusted to my grandfather. You know the Ganymedan debtor's law?"

He did indeed! It was one of the most infamous laws of the Universe: ruling that the debts of the father descended to the children and their children's children until paid.

"My family is now poor," she went on. "For a century or more we have striven to pay off the debt caused by the loss of those state funds. That's the way matters stood when I received a letter from my brother Tom in Chicago, who was employed in the office of Helgers' legitimate importing business, little aware of the smuggling. Tom had somehow got wind of the near discovery of Halkon's treasure, and I saw a chance to get a part of it by joining Helgers' party. He might not want us, but he would be practically forced to take us to keep our mouths shut. I felt that we were honestly entitled to a part of that treasure which had been stolen from our family, and with it we could pay off that old debt that had ridden our family like an Old Man of the Sea for more than a century.

"Getting into the expedition proved much simpler than I had expected. When Tom told Helgers about me he was very eager to help us—he is one of those men who is always anxious to help a girl if he thinks she is good-looking enough. So you see when I held you up in your stateroom I was merely performing my part of the scheme, although I didn't know then that Helgers had already slain the old Martian and leaped out into space.

"After that the Osprey—the ship down there on the plateau—overhauled the Western Star and took us off, and shortly afterward I learned most unpleasantly that Helgers had no intention of giving Tom and me our share unless I gave myself to him in exchange. I told Tom, and trouble started. It came to a head yesterday and there was a fight and—and Helgers killed Tom."

She began to weep quietly. Penrun stared grimly down at the black, motionless ship. Presently the girl resumed her story.

"I managed to get the air-lock open and escaped from the ship. Then that horrid spider caught me. You know the rest."

Her voice trailed off. Penrun remained silent for a while.

"You haven't even told me your name," he reminded her gently.

"Irma Boardle," she replied with a wan smile.

"I am Dick Penrun, in case you don't already know me. Captain Halkon was my grandfather. We always tried to keep the knowledge of it a family secret, since we were ashamed of it. If I—we get our hands on that treasure, I can promise you that the debt hanging over your family shall be paid first, Miss Boardle."

"Not Miss Boardle. Call me Irma," she said, the wan smile growing suddenly warm.

Penrun looked at her thoughtfully.

"But we aren't near the treasure yet," he said. "Between the spider monsters and the human monsters in the ship, our chances are rather slim. We'll just have to wait until we get a break."

As the day wore on there was a note of menace in the silence that hung over the Trap-Door City. It was nothing tangible, unless it was the appearance of two long silvery rods mounted on the top of the huge cocoon-palace of the Queen aiming down at Helgers' ship. Penrun could have sworn they were not there yesterday. The sight of them made him uneasy.

Helgers must have interpreted the silence differently, for presently a man emerged from the ship, protected against the heat by a clumsy space-suit. He hesitated, then walked slowly away from the ship, and paused again, waiting for the spiders to attack. Not a movement was made in the city. Presently he moved on again toward the cataract which had dwindled in the heat of the day to a mere trickle of hot water down to the pool in the gorge more than half a mile below.

After a time the man reached the cataract. He descended the short path that led down under the lip of rock to another ledge a few feet below it. The entrance to the Caves opened out onto this lower ledge. Little wonder, thought Penrun, that no one knew where the Caves were.

Some time later two other men from the ship followed him.

"Fools!" muttered Penrun, following them through his glasses. "They think the spiders are afraid of their ray artillery. I'll bet the monsters are either waiting until all the men wander out of the ship, or else they're getting ready to spring some hellish surprise."

Other men came out of the ship, carrying rock drills, a roll of cable and a powerful little windlass. Instead of going to the Caves, they went round the ship to the other side under the doubtful protection of the ray-guns, and sank two shafts into the granite. Into these they drove steel posts and anchored the windlass. One end of the cable was attached to the windlass and the other to the nose of the ship. Then they slowly dragged the big craft across the plateau on rollers from the ship's store room.

"That's strange!" exclaimed Penrun. "The ship can't rise! I wonder what's wrong, and why they are pulling it away from instead of toward the Caves."

"I don't know what's the matter with the ship, but I believe I know why they are moving it," volunteered Irma. "They're taking it to that hiding-place I told you Helgers picked out—there behind that upthrust of rock. You see, they think you know where the Caves are because you have explored Titan, and they think you will come directly here, so they want the ship hidden to make sure you land."

Half a hundred men in their space-suits toiled like ants about the big cylindrical craft until they at last jockeyed it into position behind the natural screen of rock. Even before it was in place other men were swarming over the ship with paint machines, coloring it a granite gray. When they had finished the ship was nearly invisible from the sky.

Penrun paid little attention to their preparations. His attention was centered on those two shining rods atop the Queen's silken palace. They now aimed at the ship in its new position. A strange idea flashed through his mind. Those rods had in some mysterious way put the elevating machinery of the Osprey out of commission!

Suppose the spiders turned them next on his own space-sphere up here on the peak? The thought sent a shudder through him. Visions of the final flight across the nightmarish, distorted granite, the running down and capture of himself and Irma, the paralyzing bite of the monsters in the cavern of the Living Dead flashed across his mind. Cold sweat stood out on his forehead. Instinctively his hand leaped to the propulsion control and hovered there.

Yet why hadn't the spiders attacked the ship, now that they had it helpless? It was not their usual tactics to give