They were not there!
"Hullo, Walt—professor! Hurry," he called back into the blackness and the foul danger he had left behind him.
To his dismay, his voice echoed hollowly upon the rocks, and went booming mysteriously down the tunnel. But human reply to his call, there was none.
With a sinking heart, Ralph realized in an instant what had happened. The professor and his companion had been overcome, by whatever it was that had emanated from the trapdoor in the tunnel.
A sort of panic seized on the boy.
He shouted and shouted, again and again, regardless of his voice being heard above. But only the mockery of the echo to his frightened cries came back to him.
It is no disparagement to Ralph to say that it required some effort on his part to nerve himself for what he did then. Summoning every ounce of resolution in his body, he threw himself on his hands and knees, with a vague recollection of having heard somewhere, that deadly gases were less deadly near to the ground.
Thus extended, the Eastern boy, with a beating heart and a dread sense of disaster oppressing him, crawled back into the danger-filled darkness from which he had just emerged.