Nearer and nearer came the mysterious sound. They could now hear it distinctly—a soft "phut-phut" on the dusty floor of the passage.
"Wow-oo, I see two eyes!" yelled Tubby, suddenly taking to his heels. His toe caught on a hidden rock, and he fell headlong in the choking dust.
Scarcely less startled than the fat boy was Rob, as he made out, glaring at them from beyond the friendly circle of light, two big green points of fire.
"Who's there?" he cried sharply.
There was no answer, but the two green globes never moved.
"Speak, or I'll fire!" cried the boy.
"A-choo-oo-o—o-o-o-o-o!"
The tense silence was shattered by a loud sneeze from Tubby, whose nostrils had become filled with the irritating dust. At the same instant an unearthly howl rang through the rocky corridors—a cry so terrible that it set Rob's heart to beating fiercely.
He pulled the trigger more by instinct than anything else, and six spurts of flame leaped from the barrel of his automatic. With a howl more ear-piercing than the first, the points of fire vanished, and there was the sound of a heavy body falling.
"Dead! whatever it is," was Rob's thought, but nevertheless he proceeded cautiously. It was well that he did so, for as he held his candle aloft, the huge, dun-colored body, which lay on the ground directly in front of him, made a convulsive spring. Rob, on the alert as he was, leaped back, and avoided it by a hair's breadth.