with a tin pail in his hand, the captain now went to the cavern of the lake. he wished very much to procure some better water than the last that had been brought, and which mok must have dipped up from a very shallow puddle. it was possible, the captain thought, that by going farther into the cavern he might find a deeper pool in which water still stood, and if he could not do this, he could get water from the little stream in the ravine. more than this, the captain wished very much to take another look at the machine by which he had let out the water. his mind had been so thoroughly charged with the sense of danger that, until this had faded away, he had not been able to take the interest in the artificial character of the lake which it deserved.

as the captain advanced into the dimmer recesses of the cavern, he soon found a pool of water a foot or more in depth, and having filled his pail at this, he set it down and walked on to see what was beyond. his eyes having now conformed themselves to the duskiness of the place, he saw that the cavern soon made a turn to the left, and gazing beyond him, he judged that the cave was very much wider here, and he also thought that the roof was higher. but he did not pay much attention to the dimensions of the cavern, for he began to discern, at first dimly and then quite plainly, a large object which rose from the bottom of the basin. he advanced eagerly, peering at what seemed to be a sort of dome-like formation of a lighter color than the rocks about him, and apparently about ten feet high.

carefully feeling his way for fear of pitfalls, the captain drew close to the object, and placed his hand upon it. he believed it to be of stone, and moving his hand over it, he thought he could feel joints of masonry. it was clearly a structure built by men. captain horn searched his pockets for a match, but found none, and he hastened back to the cave to get the lantern, passing, without noticing it, the pail which he had filled with water. he would have brought the lantern with him when he first came, but they had no oil except what it contained, and this they had husbanded for emergencies. but now the captain wanted light—he cared not what might happen afterwards. in a very short time, with the lantern in his hand, which lighted up the cave for a considerable distance about him, the captain again stood at the foot of the subterranean dome.

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.

"i have been looking for you everywhere, captain," said he. "what have you been after? more water? and you took a lantern to find it, eh? and you have been ever so far into the cave. why didn't you call me? let me have the lantern. i want to go to explore."

but the captain did not give him the lantern, nor did he allow him to go to explore.

"no, sir," said he. "what we've got to do is to hurry outside and help get supper. we must wait on ourselves to-night."

when supper was over, that evening, and the little party was sitting out on the plateau, gazing over the ocean at the sunlit sky, mrs. cliff declared that she wished they could bring their bedding and spread it on the ground out there, and sleep.

"it is dry enough," she said, "and warm enough, and if there is really nothing to fear from animals or men, i don't want ever to go inside of those caves again. i had such horrible fears and ideas when i was sitting trembling in those dismal vaults, expecting a horde of human devils to burst in upon us at any moment, that the whole place is horrible to me. anyway, if i knew that i had to be killed, i would rather be killed out here."

the captain smiled. "i don't think we will give up the caves just yet. i, for one, most certainly want to go in there again." and then he told the story of the stone mound which he had discovered.

"and you believe," cried mrs. cliff, leaning forward, "that it is really the tomb of an ancient king?"

"if it isn't that, i don't know what it can be," said the captain.

"the grave of a king!" cried ralph. "a mummy! with inscriptions and paintings! oh, captain, let's go open it this minute, before those blackies get back."

the captain shook his head. "don't be in such a hurry," he said. "it will not be an easy job to open that mound, and we shall need the help of the blackies, as you call them, if we do it at all."

"do it at all!" cried ralph. "i'll never leave this place until i do it myself, if there is nobody else to help."

miss markham sat silent. she was the only one of the company who had studied the history of south america, and she did not believe that the ancient inhabitants of that country buried their kings in stone tombs, or felt it necessary to preserve their remains in phenomenal secrecy and security. she had read things, however, about the ancient peoples of this country which now made her eyes sparkle and her heart beat quickly. but she did not say anything. this was a case in which it would be better to wait to see what would happen.

"captain!" cried ralph, "let's go to see the thing. what is the use of waiting? edna and mrs. cliff won't mind staying here while you take me to see it. we can go in ten minutes."

"no," said mrs. cliff, "there may be no danger, but i am not going to be left here with the sun almost down, and you two out of sight and hearing."

"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."

"this isn't stone in the middle of the slab," said ralph. "it's a lot more resinous stuff. i had the lantern over it and did not see it. let's take it out."

there was a circular space in the centre of the stone, about eight inches in diameter, which seemed to be covered with resin. after a few minutes' work with the jack-knives this substance was loosened and came out in two parts, showing a bowl-like depression in the slab, which had been so cut as to leave a little bar running from side to side of it.

"a handle!" cried ralph.

"that is what it is," said captain horn. "if it is intended to be lifted, i ought to be able to do it. move down a little with the lantern, and give me room."

the captain now stood on the top of the mound, with the slab between his feet, and stooping down, he took hold of the handle with both hands. he was a powerful man, but he could not lift the stone. his first effort, however, loosened it, and then he began to move it from side to side, still pulling upward, until at last he could feel it rising. then, with a great heave, he lifted it entirely out of the square aperture in which it had been fitted, and set it on one side.

in an instant, ralph, lantern in hand, was gazing down into the opening. "hello!" he cried, "there is something on fire in there. oh, no," he added quickly, correcting himself, "it's only the reflection from our light."