"What's that loud noise?" asked Tad.
His ears had caught a booming roar that was a new note in the terrifying sounds of the underworld through which they were traveling.
The boys started uneasily.
"It's water," shouted the guide. "A cataract in an underground water course. These courses have cut channels all through the limestone rocks in the Ozark Uplift."
This somewhat calmed the nerves of the lads, though not wholly so. Faster and faster rolled the car and louder and louder grew the roar of the cataract.
"Are we almost out of here?" demanded Walter uneasily.
"Yes. We shall be clear of it in five or six minutes now. You notice that we strike little grades occasionally, which cause the car to slow down considerably and for that reason the journey seems longer than it really is."
"If we have slowed down at any time I have failed to observe it," laughed Tad.
"What if we should jump the track in here?" suddenly suggested Ned.
"But we won't," answered the guide. "We—"