“Now,” he said, when they had eaten, “you have something to tell me.  Go ahead.”

They related the incident, which lost nothing of its repulsiveness by the relation: 

“And you saw no one.”

“No one alive, but I believe there was trickery.  There must have been,” said Compton, with knit brows.

“I think so too, but the trick was horrible enough to produce the effect desired.  I must say I felt a creepy sensation when I was down there yesterday.”

“But we saw no one,” said Venning, with a shudder.

“By Jove!  I forgot this;” and Compton produced a fragment of cloth.  “I took that from a post in the pool.”

“A bit of rag,” said the Hunter.

“Yes; but a bit torn out of my sleeve yesterday over there in the defile.”

Venning snatched at it.  “I have it,” he shouted.

“I see you have; but you need not yell.”

“The blind river!  It comes out under the pool!”

Compton stared.

“What do you mean?” asked Mr. Hume.

“Why, sir, we dropped a tree stump into the opening which swallows the river over there.  As it slipped from our hands, it caught Dick’s sleeve, tearing out the bit of cloth, and nearly taking him down too.”

“Well, what then?”

“Why, the stump turns up in the pool a thousand feet below, and so must the river!  You see, after entering the fissure, it twists back underground, to emerge down there at the bottom of the cliff.”

“Of course,” said Compton, eagerly; “and that body must have followed the same course.”

“Exactly.”

“That accounts for the appearance of the pool and of the dead man, but it does not explain the trickery.”

“Perhaps it does,” said Venning, who, now that he saw a cause for things, recovered his nerve and his spirit.  “There is a subterranean passage.  The formation here is volcanic.  The valley is an extinct crater, the hills are the walls.  Well, in volcanic formations, there are usually enormous caverns.  Now, then, how do we know that the Okapi has not been taken into one of those caverns opening on to the pool?”

“Good; go on to the trickery.”

“The person who hid the boat, if it is hidden, would probably be on the watch to scare off any who tried to find out what had become of it.  Well, then, if we admit that, it is easy to admit the rest—­ that a good swimmer could play the trick played on us.”

“Let me find him,” said Compton, angrily.

“Yes—­yes,” muttered Mr. Hume; “there’s a lot in that, and we’ll follow it up, but not without a good plan.”

He filled his pipe, and stared into the fire for some time.

“Clearly,” he said, “what we should do first is to find out if any one leaves the valley for the pool.  As far as we know, there is the gorge up which we came, but there may be openings direct from the valley into the underground passages.  We will leave the pool alone, as if we had had enough of it, and examine the interior cliffs.”