They were now passing along the bed of a small stream which flowed into the lake. Presently before them arose a beautiful waterfall, twenty feet high and eight or nine feet broad.
"That's the dure av me castle," announced Stoodles. "Make a quick sthep inside an' ye'll not git overly wet."
With this he dove straight into the waterfall and disappeared from view.
"Gracious! What does that mean?" came from Dave.
"He is mad and has committed suicide," muttered the doctor.
"Perhaps not," came from Bob. "That waterfall may conceal the entrance to a cave."
"Hurrah! I believe you are right," answered Dave. "And I am for finding out," and he took a step forward.
But the captain caught the young diver by the shoulder.
"Don't be rash, lad. It may cost you your life."
Scarcely had Captain Broadbeam spoken when Stoodles reappeared through the falling sheet of water.