“what is it? tell us!” exclaimed jack, almost in his last breath, for, a few seconds later he too toppled over senseless. then washington went down, while andy, bill and tom succumbed to the terrible heat.

mark felt his head swimming. his eyes were almost bulging from their sockets. he dimly remembered trying to force himself to go to the storeroom and see what was there. he started toward it with that intention, but fell half way to it.

as he did so he saw something which impressed itself on his mind, half unconscious as he was.

the door of the storeroom suddenly opened, and from it came a giant shape, that seemed to expand until it filled the whole of the apartment where the stricken ones lay. it was like the form of some monster, half human, half beast. mark shuddered, and then, closing his eyes, he felt himself sinking down into some terrible deep and black pit. a second later the whole ship was jarred as though it had hit something.

how long he and the others remained unconscious mark did not know. he was the first to revive, and his first sensation was one as though he had slept hard and long, and did not want to get up. he felt very comfortable, although he was lying flat on the floor, with his head jammed against the side of a locker. it was so dark that he could not distinguish his hand held close to his face.

“i wonder if i’m dead, and if all the others are dead too,” he thought to himself. “what has happened? let’s see, the last i remember was some horrible shape rushing from the storeroom. i wonder what it could have been? surely that was not the secret the professor referred to.”

mark shuddered as he recalled the monster that seemed to have grown more terrible as each second passed. then the boy raised himself up from his prostrate position.

“well, at any rate, some one has turned off the heat,” he murmured. “it’s very comfortable in here now. i wish i could strike a light.”

he listened intently, to learn if any of the others were moving about. he could hear them breathing, but so faintly as to indicate they were insensible. mark stretched out his hand and felt that some one was lying close to him, but who of the adventurers it was he could not determine.

“if only the dynamo was working we could have light,” he said. “but it seems to have stopped,” and, indeed there was a lacking of the familiar purr and hum of the electrical machine. in fact none of the apparatus in the ship was working.

“the storage battery!” exclaimed mark. “that would give light for a while, if i can only find the switch in the dark.”

he began crawling about on his hands and knees. it was so intensely black that he ran into many things and received severe bruises. at last he came to a doorway, and as he did so his hand came in contact with an easy chair. it was the only one aboard, and by that he knew he had passed into the sitting room. he had his general direction now, and knew if he kept straight on he would come to the engine room. there he was familiar enough with the apparatus and levers to be able to turn the electric switch.

crawling slowly and cautiously, he reached the room where all the engines were. then he had to feel around the sides to locate the switch. at length he found it. there was a click, a little flash of greenish fire, and the copper conductors came together, and the ship was flooded with the glow from the incandescents.

mark hurried back to where the others were lying. they were still unconscious, but an uneasy movement on the part of jack told that he was coming out of the stupor. mark got some ammonia and held it beneath his comrade’s nose. the strong fumes completed the work that nature had started and jack opened his eyes.

“where am i? what has happened? are any of them dead?” he asked quickly.

“i hope no one is dead,” mark replied. “as to the other question, i can’t answer. i don’t know whether we are a thousand miles underground, or floating on the ocean, though i’m more inclined to the former theory. but never mind that now. help me to bring the others back to their senses. i’ll work on the professor and you can begin on bill or tom. washington seems to be all right,” for at that moment the colored man opened his eyes, stared about him and then got up.

“i thought i was dead for suah!” he exclaimed.

“some of the others may be if we don’t hurry,” said mark. “get to work, wash!”

with the colored man to help them the two boys, by the use of the ammonia, succeeded in reviving bill, tom and old andy. but the professor, probably on account of his advanced age, did not respond so readily to the treatment. the boys were getting quite alarmed, as even some of the diluted ammonia, forced between his lips, did not cause him to open his eyes, or increase his heart action.

“if he should die, and leave us all alone with the ship in this terrible place, what would we do?” asked jack.

“he’s not going to die!” exclaimed mark. “here i have another plan. washington bring that medical electrical battery from the engine room.” this was a small machine the professor had brought along for experimental purposes.

quickly adjusting it, mark placed the handles in the nerveless fingers of mr. henderson. then he started the current. in about a minute the eyelids of the aged inventor began to quiver, and, in less than five minutes he had been revived sufficiently to enable him to sit up. he passed his hand across his forehead.

“what has happened?” he asked in a faint voice.

“i don’t know; none of us knows,” mark answered. “we all lost our senses when it got so hot, and there seemed to be some peculiar vapor in the air. the last i remember was seeing some horrible shape rush from the storeroom, soon after the ship struck. then i fainted away. when i woke up i managed to turn the lights on, and then i came back here.”

“i wonder where we are,” the old man murmured. “i must find out. we must take every precaution. washington, go and look at the gage indicating our depth.”

the colored man was gone but a few seconds. when he returned his eyes were bulging in terror.

“what is it?” asked mr. henderson, who, thanks to the battery, had almost completely recovered.

“it ain’t possible!” gasped washington. “i’ll never believe it!”

“what is it?” asked mr. henderson, while the others waited in anxiety for the answer.

“we’re five hundred miles down!” declared washington.

“five hundred miles!” muttered the inventor. “it does not seem possible, but it must be so. we fell very rapidly and the terrible draught sucked us down with incredible rapidity. but come, we must see what our situation is, and where we are. we are stationary, and are evidently on some solid substance.”

they all felt much recovered now, and, as the terrible fright of being consumed in a fiery furnace had passed, they all were in better spirits.

at the suggestion of the professor, the boys and washington made a tour of the ship. they found, for some unaccountable reason, that nearly all the engines and apparatuses were out of gear. in some the parts had broken, and others were merely stopped, from the failure of some other machine, on which they were dependent.

“i’m afraid this is the end of the mermaid,” said mark, in a sorrowful tone.

“nonsense!” replied jack, who was of a more cheerful nature. “things are not so bad as they look. the professor can fix everything.”

“i’m sure i hope so,” mark went on, not much encouraged, however, by jack’s philosophy. “it would be no joke to have to stay five hundred miles underground the rest of our lives.”

“you don’t know,” retorted jack. “don’t judge of a country you’ve never seen. this may be as fine a place as it is on the surface of the earth. i want a chance to see it,” and jack began to whistle a cheerful tune.

they completed the tour of the ship, and found, that, aside from the damage to the machinery, the mermaid had not sustained any harm. the hull was in good order, though of course they could not tell about the gas holder. it was not possible to see this except by going into the conning tower or out on the small deck, and this they did not venture to do. the connections between the holder and the main ship seemed to be all right, and there was still a small quantity of gas in the big tank, as mark found on opening a stop-cock.

they went back to the professor and told him what they had observed. he seemed somewhat alarmed, the more so as the experience he had just passed through had weakened him considerably.

“i hope i shall be able to make the repairs,” he said. “it is our only hope.”

as he spoke he looked up at the electric lights that shone overhead from wall brackets.

“who is shutting down the power?” he asked.

“there is no power on, professor,” replied mark. “i am running the lights from the storage battery. but something is the matter, for they are growing dim.”

the filaments were now mere dull red wires, and the ship was being shrouded in gloom again.

“the battery is failing!” exclaimed mr. henderson. “we shall be left in darkness, and there is no other way to produce light. i ought to have brought some lamps or candles along in case of emergency.”

the next instant the mermaid became as black as egypt is popularly supposed to be, and something like an exclamation of terror came from the professor.

for several minutes they all sat there in the blackness and gloom, waiting for they knew not what. then, suddenly, there sounded throughout the ship, a creaking as of metal sliding along metal. some big lever creaked, and, a second later the whole place was flooded with light.

“what has happened?” cried the professor, starting to his feet in alarm.

“we are going to be burned up!” exclaimed old andy.

“it’s all right! it’s all right!” yelled washington from the engine room where the boys had left him. “don’t git skeered! i done it! i opened the port holes, by yanking on the lever. golly, but we’s arrived at the new land! look out, everybody!”