"You never heard it before?" queried Grace.

"I certainly never did!"

"Say! I ho-ope I'll never hear it again," declared Bess.

But her hope did not come true. Almost immediately the prolonged
subterranean murmur echoed and reechoed through the cavern, dying
away at last in a choking sound that frightened the quartette of
girls deplorably.

Grace began to sob. Nan and Bess were really frightened dumb for
the time. Rhoda Hammond felt that she should keep up their courage.

"Don't, Gracie. Don't get all worked up. There must be some
sensible explanation of the sound. It is nothing that is going to
hurt us--"

"How do you know?" demanded Grace.

"Because, if it was any animal that might attack us, it surely
would have come nearer. And it hasn't. Besides, if it were a
dangerous beast, the ponies would have shown signs of uneasiness
long since."

In fact, this was a very sensible statement, and Nan Sherwood, for
one, quite appreciated the fact.

"Of course you are right, Rhoda. We are in no danger."