"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.
The ice had melted there to such an extent that only a thin glaze
covered the surface. Through this transparent covering Frank
Sheldon caught sight of what seemed to be the outline of a door
covered with gravel that only partially concealed it. He thought
he saw something too that faintly resembled an iron ring.
"A trap door!" he muttered under his breath, jubilant in the
thought that he had perhaps fallen upon a clue to the mystery that
had so long perplexed him.
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