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164 "That's awful!" whispered Leander, as he drew back. "That must lead to the center of the earth."

"Nonsense, Leander," replied Dick. "But it's pretty deep, I admit."

"Are there many such holes?" asked Menden.

"So many," the guide held up the fingers of both hands. "But dis is de big one. Everybody afraid of him;" and he walked on, unwilling even to remain in the vicinity.

From a distance came the sound of falling water, and presently they came to a tiny waterfall. Below was a pool of inky blackness and a small underground watercourse, which disappeared under the wall of the cavern a hundred yards away.

"I reckon dis knocks out de Mammoth Cave uf Kentucky," said Danny. "Why yer could build a hull city down here, dat's wot yer could!"

They had now reached the end of one branch of the cave, and turned back to try another branch. Soon they reached a point where the flooring was very wet and the drops of water came down steadily.

"We under ribber now," explained Remora. "Some day water come down in big heap and fill cave."

"Where does this water go to?" asked Don.