They watched the troubled surface of the lake for several minutes.
The boiling grew less and less until finally it ceased altogether, nothing having appeared.
“There you are,” said Charley, triumphantly. “Just as I said.”
“Your idea is, I suppose,” said Dick, “that the water is running off into the underground lake?”
“My idea is,” said Charley, “that under the mountain on the left here is a big cavern at a lower level than this lake and that between it and the lake is a deep hole. When the hole is full of water it discharges into the cavern gradually by a small outlet, when it gets down to a certain level the water of Izard Lake runs down into the hole until the equilibrium is restored.”
“Right,” said Dick. “Just what I think, exactly.”
“It’s too deep for me,” said Doctor Dan, shaking his head.
“What, the theory or the hole?” laughed Dick.
“Both. I should have to see the hole before I believed in it and I don’t understand the theory of all.”
“If we could only stay here and watch we would find that the boiling takes place at regular intervals and continues just so long,” added Charley. “We can investigate this later on if you say so, Dick.”