"I had to keep up appearances," returned Charlie modestly. "We started along the other passage, and presently it began to ascend, and was littered and partly blocked with boulders; finally, after much trouble and squeezing, we got up to where you found us. It was dark when we got there, but I knew by the fresh air coming in that there were some cracks somewhere leading to the upper world, and I guessed by the smell that we must be in the neighbourhood of the old cave. We went back a bit and lay down to sleep, and when I woke up we came here again so as to be ready to try and get out at the first dawn."
"Thank goodness it's all over," said Morton; "for I've had a rare fright, and Brown and I have been travelling all night. However, we won't go back without investigating the mystery of the noise."
There was still some water lying about in the rock holes around the crater, so when they returned they set to work and got breakfast ready.
Charlie thought with them that the strange noise was made by an escape of water or air, both from the regularity of the sound and its peculiar nature.
CHAPTER XIII.
Investigation of the Mysterious Noise—The Trip South—Natives Exterminated—Stuart's Initials Found.
As soon as the meal was finished, Morton, Brown, and Charlie descended the hole, but Billy declined the invitation extended to him. By the aid of the surcingles they climbed down into the passage already traversed by Charlie and Billy, and which appeared to have been an underground water-course at the time when the boiling springs were at work.
At last they arrived at the branch passage from whence came the mysterious noise, and along this they proceeded cautiously. Suddenly across their path extended a black chasm, bringing them to a stand-still. Testing the ground carefully, they crawled to the edge and looked over. The darkness was intense, but on holding the candle out, a tiny spark was reflected down below, as though from the surface of a sheet of water. Suddenly this disappeared, and the loud sigh, or, as Charlie had called it, "a regular bellow", came up from the pit. This died down, and they heard a repeated swishing noise, like water splashing against rocks. Morton inverted his candle so as to get the wick well alight, and then dropped it down the hole. They watched it falling for some time ere it struck the water with a distinctly audible hiss.
"By Jove! it's a long way down there. I don't think we need stop here any longer unless Charlie wants to go down."