“Ah!” exclaimed the learned man, as he saw this; “this cube I found evidently came from here. Let us see if we can find where it belongs.”
Leo and Dick quickly produced their knives and began cutting away the vines, while the professor put on his glasses, preparatory to making the examination.
They cleared away all around the base, which was about eighteen feet square, and just as they finished, Dick’s eye lit upon one of the places where a cube was missing.
“Here is the spot,” said he. “Now, professor, let’s see if the one you have fits here.”
The professor stepped forward and produced the cube from his pocket.
He was just about to place it in the opening when a rifle shot rang out close at hand, followed by the baying of a dog.
This so startled the man of science that he made an involuntary move forward, thrusting the cube, as he did so, squarely into the hole.
Almost instantly a hidden door flew noiselessly open, revealing a flight of stone steps, leading downward into the bowels of the earth.
A simultaneous cry of surprise left the lips of the swamp explorers as this remarkable occurrence took place.
They gazed into the opening for the space of a minute and no one spoke a word.