"The City of Aava," murmured Thor. "This is the place they sang the song to the urn, the city the Discoverer showed me." His fingers tightened on the ruby. He turned it carefully. He was back with Slag. The dwarf-man was whining, and looking around him like a scared dog until he saw Thor looming massive in the fire-flames. He grinned and came close, shaking his club. "You go where Karola went? Where the men come from?" "Yes. It's a different world, Slag, but the same. I've a feeling this ruby with the green fire in it is some sort of passport, or key, that unlocks the path into Aava's realm. It's a physical manifestation of a geometry Euclid never got around to. Dimensional worlds." Slag grunted. "We go after her?" "Take hold of the ruby. That's it." Their hands held the warm jewel. Slowly they turned it. Darkness and coldness, and dizziness, and Slag and Thor found rock under their feet, and a white moon high above them. Thor hunted for and found a narrow path of rock that twisted from the escarpment and curved downwards toward the valley. He called to Slag and they trotted Indian fashion along it. The walls of the elfin city loomed gigantic as they crossed the sandy plain that stretched for miles in front of it. Cyclopean stones were fitted one on another until they spread up and up, seemingly toward the stars themselves. Thor felt like a midge about to attack a mastodon. His feet kicked endless grains of dust walking along that massive barrier. Mile after mile they trudged, and found no gates. Slag said, "How get in?" Thor put a hand in his frayed pocket and drew out the warm ruby. He said, "This must be the only key. We haven't found any door yet." They put hands on the jewel and moved it. They went forward over the red grasslands for a hundred feet. Thor said, "This ought to be just about right." Once more they turned the jewel, and experienced the dark, the coldness, and the vertigo. Cobblestones underfoot, and smooth rock walls lining the streets as they crept forward. It was a dead city lying under the white moon, stark in its emptiness, sorrowful in its brooding strength. The windows were dark, the doorways shadowed. Once Thor and Slag heard footsteps, but they came from a great distance, and soon faded into the eternal silence. Ahead of them loomed the temple with the golden dome, where the paean to Aava had thundered forth, where the urn that held the green flame stood on its white pedestal. "They will have taken Karola there, to Aava," whispered Thor. "That is where we must go. To the temple of the green flame." A massive knob of bronze, covered with greenish rot and carved with the emblem of Aava-in-the-urn, screeched as Thor turned it. The thick oaken door swung wide. Pale radiance bathed the arched columns that trod the mosaic floor of the vast chamber. At the far end of the room, the black urn stood empty and black.