They had come perhaps to within twenty feet of the rear wall, when
Frank gave a sudden exclamation.
CHAPTER XI
MARSHAL FOCH AND GENERAL PERSHING
"What is it, Frank?" asked Bart Raymond in a low voice.
"My foot sank in," explained Frank. "It's softer here for some reason than in the rest of the alley. Just wait a minute till I can feel around here and see what I can make of it."
He felt about cautiously with his feet and found that the ice had softened for a space of about four feet and that the space was almost perfectly square.
"There's some reason why this spot should be different from the rest," he said, after having verified his discovery. "It's all open to the weather, like every other part of the alley, and there's only one explanation. There's heat coming up from beneath. That means that there must be an open space beneath this spot. I wish I dared use my flashlight."
"Wait a minute," said Bart. "I'll slip out to the mouth of the alley and see if the coast is clear. If it is, I'll give a low whistle and then you flash your light and see what it shows you."
He left his companion, and a moment after Frank heard the signal agreed upon.
Instantly he flashed his light on the rectangular space that had caught his attention.