A hundred echoes followed the report, and then an answering shot was heard.
The next instant all four darted into the passage.
They had hardly taken a dozen steps when their heels flew from beneath them, and they went shooting down a slippery decline with the speed of the wind.
But in a very few seconds they brought up in a confused heap on a rocky floor.
"Thunder!" ejaculated Lank Edwards. "What has happened now?"
He scrambled to his feet, followed by his companions.
Dr. Pestle's torch had become extinguished, and he hastened to light it again.
As the feeble flame flared up, they found that they were in what appeared to be a tomb.
All around them they beheld piles of bones and grinning skulls.
The professor's teeth began to chatter.