But this was not all.
The Antarctic natives had entered the place, but none of them were in sight.
Frank was in a position whence he could easily view the whole interior of the place.
But an explanation of their disappearance was easily obtained.
Just beyond the lava basin there was a dark, cavernous opening which appeared to trend downward.
Frank understood it all at once.
“I have it!” he cried. “This is only one of many caverns in this volcanic range. The whole region here doubtless is honeycombed by the action of currents of lava. Doubtless their retreat is deep down in the bowels of the earth.”
Captain Hardy heard this with dismay.
“Then we can never hope to rout them out!” he said. “That will not be possible.”
“On the contrary, I believe it is possible,” said Frank.