"It seems highly and increasingly probable to me," the engineer said, after long thought, "that we have here the actual cause of the vast blight of death that left us two alone in the world. I rather think that at the time of the great explosion which produced this rent, certain highly poisonous gases were thrown off, to impregnate the entire atmosphere of the world. Everybody must have been killed at once. The poison must have swept the earth clean of human life." "But how did _we_ escape?" asked the girl. "That's hard telling. I figure it this way: The mephitic gas probably was heavy and dense, thus keeping to the lower air-strata, following them, over plain and hill and mountain, like a blanket of death. "Just what happened to us, who can tell? Probably, tightly housed up there in the tower, the very highest inhabited spot in the world, only a very slight infiltration of the gas reached us. If my theory won't work, can you suggest a better one? Frankly, I can't; and until we have more facts, we've got to take what we have. No matter, the condition remains--we're alive and all the rest are dead; and I'm positive this cleft here is the cause of it." "But if everybody's dead, as you say, why hunt for men?" "Perhaps a handful may have survived among the highlands of the Rockies. I imagine that after the first great explosion there followed a series of terrible storms, tornadoes, volcanic eruptions, tidal waves and so on. You remember how I found the bones of a whale in lower Broadway; and many of the ruins in New York show the action of the sea--they're laid flat in such a manner as to indicate that the island was washed on one or two occasions by monster waves. "Well, all these disturbances probably finished up what few survivors escaped, except possibly among the mountains of the West. A few scattered colonies may have survived a while--mining camps, for instance, or isolated prospectors, or what-not. They may all have died out, or again, they may have come together and reestablished some primitive form of barbarous or even savage life by this time. There's no telling. Our imperative problem is to reach that section and explore it thoroughly. For there, if anywhere, we'll find survivors of our race." "How about that great maelstrom that nearly got us?" asked the girl." Can you connect that with the catastrophe?" "I think so. My idea is that, in some way or other, the sea is being sucked down into the interior of the earth and then hurled out again; maybe there's a gradual residue being left; maybe a great central lake or sea has formed. Who knows? At any rate all the drainage system of the country seems to have been changed and reversed in the most curious and unaccountable manner. I think we should find, if we could investigate everything thoroughly, that this vast chasm here is intimately connected with the whole thing."