“Let us all go,” said Edna.

The captain considered for a moment.  “Yes,” said he, “let us all go.  As we shall have to take a lantern anyway, this is as good a time as another.”

It was not an easy thing for the two ladies to get over the wall at the end of the passage, and to make their way over the rough and slippery bottom of the lake basin, now lighted only by the lantern which the captain carried.  But in the course of time, with a good deal of help from their companions, they reached the turning of the cave and stood before the stone mound.

“Hurrah!” cried Ralph.  “Why, captain, you are like Columbus!  You have discovered a new hemisphere.”

“It is like one of the great ant-hills of Africa,” said Mrs. Cliff, “but, of course, this was not built by ants I wonder if it is possible that it can be the abode of water-snakes.”

Edna stood silent for a few moments, and then she said, “Captain, do you suppose that this dome was entirely covered by water when the lake was full?”

“I think so,” said he.  “Judging from what I know of the depth of the lake, I am almost sure of it.”

“Ralph!” suddenly cried Mrs. Cliff, “don’t try to do that.  The thing may break under you, and nobody knows what you would fall into.  Come down.”

But Ralph paid no attention to her words.  He was half-way up the side of the mound when she began to speak, and on its top when she had finished.

“Captain,” he cried, “hand me up the lantern.  I want to see if there is a trap-door into this affair.  Don’t be afraid, Mrs. Cliff.  It’s as solid as a rock.”

The captain did not hand up the lantern, but holding it carefully in one hand, he ascended the dome by means of the row of protuberances on the other side, and crouched down beside Ralph on the top of it.

“Oh, ho!” said he, as he moved the lantern this way and that, “here is a square slab fitted into the very top.”

“Yes,” said Ralph, “and it’s got different mortar around the edges.”

“That is not mortar,” said the captain.  “I believe it is some sort of resin.  Here, hold the lantern, and be careful of it.”  The captain took his jack—­knife out of his pocket, and with the large blade began to dig into the substance which filled the joint around the slab, which was about eighteen inches square.  “It is resin,” said he, “or something like it, and it comes out very easily.  This slab is intended to be moved.”

“Indeed it is!” exclaimed Ralph, “and we’re intended to move it.  Here, captain, I’ll help you.  I’ve got a knife.  Let’s dig out that stuff and lift up the lid before the darkies come back.  If we find any dead bodies inside this tomb, they will frighten those fellows to death, if they catch sight of them.”

“Very good,” said the captain.  “I shall be only too glad to get this slab up, if I can, but I am afraid we shall want a crowbar and more help.  It’s a heavy piece of stone, and I see no way of getting at it.”