"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.