They followed the “splinter trail,” as they called it, until it came to an end right where the two sides of the big stony V came together.
“Here ought to be the door—here or hereabouts,” the professor said as he drew a geologist’s hammer from his pocket, for he was a geologist as well as a botanist and a “bugologist.”
He began to tap gently on the walls of the defile. They were of rough stone, and so cunningly had the concrete coating been made for the wooden door that it could not be detected by an difference in hue or texture.
But suddenly the hammer, instead of giving back a sharp, thudding sound, produced a hollow boom.
“There it is!” cried Jerry.
“Right,” assented the scientist. “And you can see the outline of the door,” and he pointed to an irregular crack starting at the floor of the gorge, rising up about five feet, always irregular, then down again until it reached the rocky floor once more, the space between being roughly shaped like an inverted U with about ten feet distance between the two points.
“But how does it open?” asked Ned. “If we can’t get through we aren’t much better off than before.”
“It is only a light wooden door, covered on the outside with expanded metal lath and that, in turn, with concrete,” said the professor. “It was made in this irregular shape so that the crack, where it fitted into the opening of the tunnel, would look like a crack in the wall. But now we know what the crack means we can pry the door open.”
Ned ran to get the necessary tools, and while he was coming back with them Jerry and Bob looked at the secret door. It was so cunningly devised that from the gorge few would have guessed its existence. They, in their previous searches, had probably stared right at the crack but uncomprehendingly.