The largest tunnel was immediately ahead. Frank, accordingly, stepped into the gloomy passage. The others followed.
"When I was going to sleep to-night, I never thought I'd wake up and take part in an exploring trip underground before morning," observed Chet.
Frank gave an exclamation.
"Here's what we were looking for!" he cried.
"What?"
"A footprint."
The others crowded around him.
Clearly discernible in the radiance of Frank's flashlight, the lads could see the imprint of a boot in a patch of wet sand on the floor of the tunnel.
"Looks like a fresh track, too," said Joe.
"We're on the right trail. Let's keep moving."