The four boys left behind gazed eagerly and admiringly at the airship with her invisible occupants for several minutes, not a word escaping the lips of any of them. Suddenly Frank broke the silence by saying:
“Say, fellows, maybe we’ll never have a chance to come up here again. Dr. Byrd won’t let us come up in the airship, and the passage through the cave’s closed. Let’s explore this mountain top some before Mr. Miles gets back.”
“There isn’t much to explore,” replied Ferd. “We can see pretty near everything standing right here.”
“We can’t see the other side of those big rocks and cliffs over there,” Byron answered, pointing to the south through the thin belt of timber: “That’s what we were lookin’ at through the field-glasses from Uncle Sam’s mountain, you know.”
“Yes, let’s go and have a look at it,” proposed Hal.
There was no need of further urging, and the four boys started off at a brisk rate. Through the timber they ran and then southward along the high ridge of rocks and mounds, until they came to a passage through the rocks. Into this passage they entered and hastened on until near the middle a new discovery brought them to a halt.
“My goodness! Look at that!”
Hal exclaimed thus as he stopped suddenly and pointed toward something very remarkable fifteen feet ahead. It was the opening of a cave in the clay and stone wall, and slightly ajar was a wooden door of rough-hewn tree trunks.
“Why, somebody lives up here!” cried Ferdinand in excited tones. “Let’s go and see who it is.”
“No,” Hal cautioned. “We’ve got to be careful. If anybody does live here, he’s probably crazy. Let’s pick up some stones to throw at him if he comes at us.”