it was but a moment’s work for frank to spring out on deck. then he reached the sands below.

swinging the ax aloft, he rushed to von bulow’s assistance.

he was not a moment too soon.

captain bell had already attacked the monster. but frank’s arrival saved the day.

the young inventor swung the ax over his head and made a slashing blow at the monster’s head.

it struck the enormous hawk-like beak and slashed off part of it.

quick as thought frank repeated the blow.

the monster writhed and made an effort to encircle frank with another of its long arms.

but the young inventor this time buried the ax to the head in the creature’s cat-like eye.

this was the telling stroke.

it penetrated the brain, and the octopus straightened in death throes.

the battle was over.

barney and pomp were watching the contest from one of the windows of the submarine boat.

“golly!” cried pomp, cutting a pigeon wing. “marse, frank am done fixed dat critter for suah!”

“bejabers! when misther frank goes for to do a thing, he does it up in illegant shape!” declared barney.

“yo’ am right, i’ish.”

prof. von bulow was extricated from the embrace of the octopus, and all returned to the boat.

the adventures detailed by the captain and the professor were thrilling, indeed, and the others listened to them with interest.

“it will be hardly safe to repeat that sort of thing!” said frank; “the next time you gentlemen go out on an exploring expedition, i think that one of us who is more experienced in that sort of thing had better go with you.”

“we shall not demur,” said captain bell, with a laugh. “i am afraid we are hardly qualified to face such risks.”

after frank had concluded his inspection of the machinery it was decided to at once continue the journey into the atlantic valley.

so the machinery was put in motion, and the boat dove into the dark depths to be lost from the world for many months.

the searchlight showed all about for a great distance as plain as day.

but the boat passed over immense depths where all was darkness far below, and into which the boat could not descend on account of the enormous pressure.

there was an automatic gauge on the pilot-house which registered this pressure and determined the depth to which it was safe to go.

below this the boat would be crushed like an egg shell.

but as a general thing the dart was enabled to keep in view of the bed of the ocean.

this was now much diversified by hills and even mountains.

there were innumerable caves, many of which were of enormous depth.

in fact the dart once sailed into one of these nearly half a mile before it was discovered by the voyagers that they were in a cave.

then, of course, it was in order to turn about and sail out, but before this was done a thrilling experience was had.

prof. von bulow was greatly interested in this wonder of the ocean depths—this submarine cave of the great atlantic valley.

“there is nothing like it on top of the earth,” he declared. “i would like very much to examine its structure, which has the appearance of being quartz.”

“quartz!” exclaimed captain bell.

“yes.”

“not of the gold-bearing species?”

“it is not impossible.”

at once everybody was interested, even frank himself.

“you shall have your desire, professor,” he declared.

accordingly the dart was brought to a stop and rested upon the bed of the cavern. the diving suits were brought out.

it was decided that frank and barney should accompany the professor this time.

pomp and captain bell were to remain on board the dart.

the searchlight illumined the walls of the cavern in beautiful style. they were of great beauty.

overhead they were crystal-like in their character, and the declaration by the professor that they were of quartz did not by any means seem far fetched.

the diving suits were donned, and the three explorers, well armed with ax and knife, left the dart.

the floor of the cavern was tolerably smooth. they easily made their way over it, and von bulow was soon at work.

it did not take the professor long to get to work with his hammer.

in a very short time he had chipped off enough of the quartz to reveal a curious yellow vein which seemed to extend an indefinite distance into the ledge.

the professor appeared excited.

seeing this, frank put his helmet close to his and shouted:

“well, von bulow, what do you make of it?”

“gold!” shouted the excited scientist.

frank was astounded.

“gold?” he repeated.

“yes.”

“a submarine gold mine?”

“exactly.”

barney had also placed his helmet close to the others and heard this.

“bejabers!” he cried; “then it’s a moighty fortune we’ve found.”

“i think it would assay heavily,” declared the professor; “if it was only on the surface, now, what it would be worth!”

frank saw the point.

“but it is of no value here?”

the professor nodded.

“exactly. how could a stamp mill be operated down here at this depth? it would cost more than the ore is worth to mine it.”

“but perhaps we might find some nuggets hereabouts.”

“no,” replied von bulow, decidedly. “it is not likely. it is probable that all the gold hereabouts is secreted in these quartz walls. it is only a question of getting it out.”

“which is impracticable.”

“exactly.”

the thought that they were really in a mighty submarine gold mine was a most thrilling one.

however, the professor had other points to gain now, so he left the gold vein and began some further explorations of the submarine cave.

this extended an unknown distance into the bowels of the earth.

the party did not venture to go far beyond the rays of the searchlight.

there would be great danger of getting lost in the labyrinth of passages, and certainly there would be no pleasure in this.

in view of the experiences of the professor and the captain, this was an issue to be avoided.

however, they carried the exploration as far as seemed safe.

this was a number of hundred yards from the submarine boat, and they began to think of returning when barney stumbled upon an exciting adventure.

suddenly he espied a curious-looking round body lying close to the wall of the cavern, and extending out of sight into dark depths.

the celt was nothing if not curious.

it looked like a strange formation of some aquatic growth, and unthinkingly barney jabbed the point of his knife into it.

the result was thrilling.

the round body instantly contracted and then rebounded, throwing barney backward with great force.

and then out of the darkness of a cavern passage came a great flat head with horrid jaws.

it was a species of sea serpent. the huge coils were thrashing the water of the cavern furiously, and the divers stood for a moment paralyzed with terror.

the sea serpent was undoubtedly the habitue of the ocean cavern. he seemed also disposed to resent this invasion upon his chosen territory.

the situation was critical.

frank saw at once that quick action must be made, or serious consequences would be the result.

the young inventor, therefore, at once signaled the others to follow him, and began a retreat for the dart.

but the sea serpent was following, and was certain to overtake them. already frank felt the creature right behind him.

and he turned to see those horrid jaws wide open above him. if they should strike him, doubtless it would be a death blow.

so frank quickly dodged and made a blow at the monster with his ax.

it missed the mark, and the next moment the creature’s jaws were right over frank. it was a horrible moment.

only the young inventor’s rare presence of mind saved him then.

quick as a flash he drove the ax into the serpent’s jaws and down its throat. then he was hurled half senseless to the floor of the cavern.

the ax disappeared down the serpent’s throat instantly.

it was undoubtedly not averse to a diet of the sort, for it did not seem to affect his snakeship.

but frank was for the nonce safe.

he had been hurled into a small recess in the wall of the cavern, and the serpent passed on.

it was now after barney and von bulow.

but frank’s fracas with it had given them a chance to get a good lead.

they were now quite near the dart, and were enabled to reach it in time.

on to the deck and into the vestibule they dashed.

the next moment the submarine boat received a terrific shock.

the sea serpent struck it full force, and for a moment it seemed as if the fate of the dart was sealed.

but, as good luck had it, the steel work of the boat was harder than the serpent’s head.

it resisted the shock. but pomp and captain bell were thrown across the cabin.

they had seen the serpent coming in pursuit of von bulow and barney, and it had given them a thrill.

“golly!” screamed pomp. “de debbil am after dem. shuah nuff dat big snake will swallow us all up!”

“great heavens!” gasped captain bell, “the creature means to strike us.”

“look out fo’ yo’sef!”

into the vestibule darted barney and von bulow.

then came the shock.

but the dart was not demolished. on the contrary the sea serpent seemed for a moment stunned.

then it was that barney recovered his senses. he remembered that frank had been left behind, and with an inward cry darted out of the vestibule.

“bejabers! it’s not mesilf as will go off and lave misther frank in such a heap av trouble,” he muttered. “i’ll niver cum back widout him!”