Just as he was about to recoil from the verge of the awful pit, there was a grinding sound as of mortar giving way, and then, wholly unable to save himself, Frank went headlong into the abyss.
Down, down he felt himself going.
He clutched at the air wildly, experienced a sense of suffocation, then he felt a sudden shock and knew that he was in water.
It seemed as if his breath must leave his body before he came up out of this water.
Coming to the surface, he drew in a deep breath, and then struck out to swim.
The water was icy cold, and he knew that its depth must be great.
Also, he believed its extent to be considerable, as the reverberations coming to his ears were far-reaching and loud.
Frank had no means of knowing into what sort of place he had fallen, or what was to be the outcome of this adventure.
CHAPTER X.
THE SEALED CHAMBER.
But he did know that he was in the embrace of some underground lake or reservoir of water.