Chet gave a cheer.
"Let's investigate."
They advanced on the cave, but when they were just in front of the entrance they halted with exclamations of surprise.
Tacked on a board stuck in the sand beside the cave-mouth was a tattered fragment of paper. On it, in black letters scrawled with a heavy pencil, they read:
NO TRESPASSING.
The boys looked at this sign in astonishment.
"By order of the chief of police," murmured Chet, with a grin.
"Looks as if somebody has been here before," Biff observed.
"Perhaps somebody just put up the sign for a joke. Let's take a peep inside."
Frank advanced toward the cave.