might once
have been rock, the boys found an opening which had been, apparently,
closed for a long period of time. When finally cleared, after an hour
of hard work, the opening from which the current of air had come was
discovered to be a door like arch in the west wall of the main chamber.

The electric flashlight, however, when introduced into the opening,
showed a narrow passage beyond the opening instead of a square room.
This tunnel-like passage was not far from six feet in width and about
that in height. The walls showed that it had been cut through solid
rock.

The boys listened for some indication of life or motion in the tunnel,
but all was silent. Not even a bird or creeping thing disturbed the
stillness of the place.

"Shall we go in now?" asked Nestor.

"Sure!" replied Shaw. "We may find a well in there!"

"Or a soda fountain, or a modern filter," grinned Jimmie. "How would
they ever get a well down through this mountain?"

"Water in wells comes from elevations before it gravitates to the
bottom of the holes from which we pump it," Shaw declared, in defense
of his suggestion. "There may be a reservoir here somewhere."

"How far is this cavern floor from the surface above it?" asked Harry
Stevens, with a judicial air.

"About four hundred feet," was the reply. "We must be about that
distance from the highest point here."

"Then there is no reason why there should not be a reservoir above us,"
said Harry. "Water would filter through these rocks, all right."

The boys passed on in a southwesterly direction to the end of the
tunnel, which was about fifty feet from the opening. Here they found a
chamber about 10x16 feet in size. At the south side of this chamber
was a trough-shaped place cut in the rock, and through this a small
rivulet of water ran.

"I knew the people who built this shop wouldn't put in their time where
no water could be procured," declared Fenton. "Why, this is simply
fort, a mountain residence, where valley people came in time of war and
secreted themselves. If we could read the hieroglyphics on the walls,
we would be able to write a history of their troubles."

"Were they the real thing in cave-dwellers?" asked Jack, who was not
noted for his studious habits, and who depended on his companions for a
knowledge of the countries he visited as a member of the Black Bear
Patrol.

"Earlier than some of the cave-men," replied Harry. "I wonder if this
water is any good