He walked around it.  He raised and lowered his lantern, and examined it from top to bottom.  It was one half a sphere of masonry, built in a most careful manner, and, to all appearances, as solid as a great stone ball, half sunken in the ground.  Its surface was smooth, excepting for two lines of protuberances, each a few inches in height, and about a foot from each other.  These rows of little humps were on opposite sides of the dome, and from the bottom nearly to the top.  It was plain they were intended to serve as rude ladders by which the top of the mound could be gained.

The captain stepped back, held up his lantern, and gazed in every direction.  He could now see the roof of the cavern, and immediately above him he perceived what he was sure were regular joints of masonry, but on the sides of the cave he saw nothing of the sort.  For some minutes he stood and reflected, his brain in a whirl.  Presently he exclaimed: 

“Yes, this cave is man’s work!  I am sure of it.  It is not natural.  I wondered how there could be such a cave on the top of a hill.  It was originally a gorge, and they have roofed it over, and the bottom of the basin has been cut out to make it deeper.  It was made so that it could be filled up with water, and roofed over so that nobody should know there was any water here, unless they came on it by means of the passage from our caves.  That passage must have been blocked up.  As for the great opening in the side of the cave, the rocks have fallen in there—­that is easy enough to see.  Yes, men made this cave and filled it with water, and if the water were high enough to cover the handle of that machine, as it was when I struck it, it must also have been high enough to cover up this stone mound.  The lake was intended to cover and hide that mound.  And then, to make the hiding of it doubly sure, the men who built all this totally covered up the lake so that nobody would know it was here.  And then they built that valve apparatus, which was also submerged, so that they could let out the water when they wanted to get at this stone thing, whatever it is.  What a scheme to hide anything!  Even if anybody discovered the lake, which would not be likely until some part of the cave fell in, they would not know it was anything but a lake when they did see it.  And as for letting off the water, nobody but the people who knew about it could possibly do that, unless somebody was fool enough to take the cold bath I was obliged to take, and even then it would have been one chance in a hundred that he found the lever, and would know how to turn it when he did find it.  This whole thing is the work of the ancient South Americans, and I imagine that this stone mound is the tomb of one of their kings.”

At this moment the captain heard something, and turned to listen.  It was a voice—­the voice of a boy.  It was Ralph calling to him.  Instantly the captain turned and hurried away, and as he went he extinguished his lantern.  When he reached his pail of water he picked it up, and was very soon joined by Ralph, who was coming to meet him over the bottom of the lake.