st have happened. I flew back to the basement
stairs, to meet Mr. Harbison at the foot. He was grimy and dusty,
with streaks of coal dust over his face, and he had been examining his
revolver. I was just in time to see him slip it into his pocket.

"What is the matter?" I demanded. "Is any one hurt?"

"No one," he said coolly. "We've been cleaning out the furnace."

"With a revolver! How interesting--and unusual!" I said dryly, and
slipped past him as he barred the way. He was not pleased; I heard him
mutter something and come rapidly after me, but I had the voices as a
guide, and I was not going to be turned back like a child. The men had
gathered around a low stone arch in the furnace room, and were looking
down a short flight of steps, into a sort of vault, evidently under the
pavement. A faint light came from a small grating above, and there was a
close, musty smell in the air.

"I tell you it must have been last night," Dallas was saying. "Wilson
and I were here before we went to bed, and I'll swear that hole was not
there then."

"It was not there this morning, sir," Flannigan insisted. "It has been
made during the day."

"And it could not have been done this afternoon," Mr. Harbison said
quietly. "I was fussing with the telephone wire down here. I would have
heard the noise."

Something in his voice made me look at him, and certainly his expression
was unusual. He was watching us all intently while Dallas pointed out to
me the cause of the excitement. From the main floor of the furnace room,
a flight of stone steps surmounted by an arch led into the coal cellar,
beneath the street. The coal cellar was of brick, with a cement floor,
and in the left wall there gaped an opening about three feet by three,
leading into a cavernous void, perfectly black--evidently a similar
vault belonging to the next house.

The whole place was ghostly, full of shadows, shivery with
possibilities. It was Mr. Harbison finally who took Jim's candle and
crawled through the aperture. We waited in dead silence, listening to
his feet crunching over the coal beyond, watching the faint yellow light
that came through the ragged opening in the wall. Then he came back and
called through to us.

"Place is locked, over here," he said. "Heavy oak door at the head of
the steps. Whoever made that opening has done a prodigious amount of
labor for nothing."

The weapon, a crowbar, lay on the ground beside the bricks, and he
picked it up an