With increasing excitement, the chums pressed forward and in a few moments Frank stepped out of the passage into another cave. This was the largest cave of all, an enormous underground vault, and even the flashlight beams failed to reveal the rocky walls and ceiling.
The floor was rough and broken fragments of rock were strewn about.
"Watch your step," warned Frank, as he made his way across the cave.
The others had flashlights and the floor was well illuminated as the boys slowly picked their way among the rocks. The far wall of the huge cavern was still invisible.
"This is a whopper!" said Joe, in an awed whisper.
Frank stopped, with a murmur of annoyance.
"What's the matter?" asked Chet.
"My flashlight. It's on the blink."
Vainly, Frank tried to coax a gleam from the refractory instrument. It was no use. He put the light in his pocket.
"I'll have to fix it to-morrow," he said. "It won't work any more to-night by the looks of things."