oment they were safely below. The
corridor was quite empty.

"When the crash came," Estelle explained, her voice shaking with
the reaction from her fear of a moment ago, "every one thought the
building was coming to pieces, and ran out. I'm afraid they've all
run away."

"They'll be back in a little while," Arthur said quietly.

They went along the big marble corridor to the same western door,
out of which they had first gone to see the Indian village. As
they emerged into the sunlight they met a few of the people who
had already recovered from their panic and were returning.

A crowd of respectable size gathered in a few moments, all still
pale and shaken, but coming back to the building which was their
refuge. Arthur leaned wearily against the cold stone. It seemed to
vibrate under his touch. He turned quickly to Estelle.

"Feel this," he exclaimed.

She did so.

"I've been wondering what that rumble was," she said. "I've been
hearing it ever since we landed here, but didn't understand where
it came from."

"You hear a rumble?" Arthur asked, puzzled. "I can't hear anything."

"It isn't as loud as it was, but I hear it," Estelle insisted. "It's
very deep, like the lowest possible bass note of an organ."

"You couldn't hear the shrill whistle when we were coming here,"
Arthur exclaimed suddenly, "and you can't hear the squeak of a
bat. Of course your ears are pitched lower than usual, and you can
hear sounds that are lower than I can hear. Listen carefully. Does
it sound in the least like a liquid rushing through somewhere?"

"Y-yes," said Estelle hesitatingly. "Somehow, I don't quite
understand how, it gives me the impression of a tidal flow or
something of that sort."

Arthur rushed indoors. When Estelle followed him she found him
excitedly examining the marble floor about the base of the vault.

"It's cracked," he said excitedly. "It's cracked! The vault rose
all of an inch!"

Estelle looked and saw the cracks.

"What does that mean?"

"It means we're going to get back where we belong," Arthur cried
jubilantly. "It means I'm on the track of the whole trouble.
It means everything's going to be all right."

He prowled about the vault exultantly, noting exactly how the cracks
in the flooring ran and seeing in each a corroboration of his theory.

"I'll have to make some inspections in the cellar," he went on
happily, "but I'm nearly sure I'm on the right track and can figure
out a corrective."