"Huh! Death rays!" he exclaimed then. "'Sa good thing for us we're essentially three-dimensional yet, or we'd probably never have known what struck us. Now let's see--where's our river? Oh, yes; over this way. Wonder if we'd better take these shields along? Guess not, they're pretty well shot--we'll pick us up a couple of good ones on the way, and I'll get you a grill like this one as a good club, too." "But there's no door on that side!" Margaret protested. "We should fret a lot about that--we'll roll our own as we go along." His heavy boot crashed against the wall before them, and a section of it fell outward. Two more kicks and they were through, hurrying along passages which Seaton knew led toward the buried river, breaking irresistibly through solid walls whenever the corridor along which they were moving angled away from his chosen direction. Their progress was not impeded. The hyperbeings were willing--yes, anxious--for their unmanageable prisoners to depart and made no further attempts to bar their path. Thus the river was soon reached. The airship in which they had been brought to the hypercity was nowhere to be seen, and Seaton did not waste time looking for it. He had been unable to understand the four-dimensional controls even while watching them in operation, and he realized that even if he could find the vessel the chance of capturing it and of escaping in it was slight indeed. Therefore, throwing an arm around his companion, he leaped without ado into the speeding current. "But, Dick, we'll drown!" Margaret protested. "This stuff must be altogether too thin for us to swim in--we'll sink like rocks!" "Sure we will, but what of it?" he returned. "How many times have you actually breathed since we left three-dimensional space?" "Why, thousands of times, I suppose--or, now that you mention it, I don't really know whether I'm breathing at all or not--but we've been gone so long--Oh, I don't believe that I really know _anything_!" "You aren't breathing at all," he informed her then. "We have been expending energy, though, in spite of that fact, and the only way I can explain it is that there must be fourth-dimensional oxygen or we would have suffocated long ago. Being three-dimensional, of course we wouldn't have to breathe it in for the cells to get the benefit of it--they can grab it direct. Incidentally, that probably accounts for the fact that I'm hungry as a wolf, but that'll have to wait until we get back into our own space again." True to Seaton's prediction, they suffered no inconvenience as they strode along upon the metaled pavement of the river's bottom, Seaton still carrying the bent and battered grating with which he had wrought such havoc in the corridor so far above.