stop, as they were anxious to get to the underground city.
"Hark! What's that?" asked Torn, when they had gone about a mile along the tunnel. "Don't you hear something?"
"Sounds like a roaring," agreed Ned. "Maybe it's more of the bats."
"Doesn't sound like bats," declared Tom. "It's more like a waterfall. Come on."
They hurried forward, the strange sound increasing at every step, until it filled the tunnel with its menacing roar.
"That's strange," said Tom in worried tones. "I hope we don't come to a waterfall."
Suddenly the tunnel made a turn, and as they went around the curve in the wall the sound smote on their ears with increased violence. Tom raced forward, focusing his electric lamp down on the stone corridor. The next instant he cried out:
"A river! It's an underground river and we can't go any further! We're blocked!"
The others came to his side, and there, in the glare of their lamps, they saw rushing along, between two walls of stone, a dark stream which caused the roaring sound that had come to them. The tunnel was cut squarely in two by the stream, which was at least thirty feet wide, and how deep they could only guess. Swiftly it flowed on, its roar filling the tunnel.
