"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."