Lead weights in their boots held them so that they could walk. Falken thrust at the rock with a steel-shod alpenstock.
"It's like glass," he said. "Some unfamiliar compound, probably, fused out of raw force in the Solar disturbance that created the planets. That would explain its resistance to heat."
Radio headphones carried Avery's voice back to him clearly, and Falken realized that the stuff of the planet insulated against Solar waves, which would normally have blanketed communication.
"Whatever it is," said Avery, "it sucks up light. That's why it's never been seen. Only little glimmers seep through, too feeble for telescopes even on Mercury to pick up against the Sun. Its mass is too tiny for its transits to be visible, and it doesn't reflect."
"A sort of dark stranger, hiding in space," said Sheila, and shivered. "Look, Eric! Isn't that a cave mouth?"
Falken's heart gave a great leap of hope. There were caves on Pluto. Perhaps, in the hidden heart of this queer world....
They went toward the opening. It was surprisingly warm. Falken guessed that the black rock diffused the Sun's heat instead of stopping it.
Thin ragged spires reared overhead, stabbing at the stars. Furtive glints of light came and went in ebon depths. The cave opened before them, and their torches showed glistening walls dropping sheer away into blackness.
Falken uncoiled a thousand-foot length of synthetic fiber rope from his belt. It was no larger than a spider web, and strong enough to hold Falken and Avery together. He tied it to each of their metal boots to and let it down.