Frank had some misgivings as to the policy of allowing these two inexperienced men to leave the Dart.

So he caused Barney to place one of the suits within easy reach, so that in case of need he could easily don it and go to their aid.

The two divers were all equipped, and all that was now necessary was for them to leave the Dart.

This did not seem such a very easy thing to do.

It would seem that to open a door or window for exit would be to instantly flood the interior of the boat.

And so it would have.

Frank had provided for this contingency, however, in the construction of the boat.

A door opened from the cabin into a vestibule. Entering this the divers closed a door behind them and opened a valve which flooded the vestibule.

Then they opened the outer doors with impunity and walked out on the deck.

The return to the cabin was effected by entering the vestibule, closing the outer door and turning another valve which expelled the water by pneumatic pressure. Then they could safely enter the cabin.

This was only one of the simplest of the many wonderful devices with which the Dart was provided.

Once out on deck the divers experienced queer sensations for a moment.

The pressure of the water for a time made them blind and dizzy.

But they soon recovered and went over the Dart’s rail.

They stood upon the bed of the ocean. It was a wonderful reflection.

About were all the wonders heretofore denied the sight of man. Truly this was no ordinary experience.

Forgetting himself, Bell attempted to speak to the professor. But the latter, of course, could not hear him.

It was only by putting their helmets together that they were able to converse, and then with difficulty.

They walked in the pathway of light from the boat.

Looking back through the plate glass windows they could easily see the interior of the Dart.

Both divers now began to enjoy themselves looking for specimens and exploring the submarine recesses.

While Captain Bell was not a scientist, he was nevertheless pleased to render aid to the professor.

Thus they kept on, gradually working further and further away from the submarine boat, until finally they reached the shadows which indicated the limit of the searchlight.

Beyond all was pitchy blackness, for it was into the unknown depths of the great Atlantic Valley.

Captain Bell put his helmet close to the professor’s and shouted:

“Is it safe to go further?”

“I think not,” replied Von Bulow. “We had better turn back.”

But even as he said this he saw a queer specimen of fish slowly make its way into a coral cave near.

“I must have that fellow,” he exclaimed, excitedly. “He is a new variety.”

Without a thought of possible peril the professor darted in pursuit. Into the cavern he went.

Bell stood and looked after him somewhat doubtfully.

The old sea captain did not reckon but that Von Bulow was amply capable of taking care of himself, though really he regarded it as a trifle risky.

The professor turned an angle of the cavern and was out of sight.

The captain was a trifle weary with the exertion of climbing over the slippery piles of seaweed, and did not follow.

He waited what seemed to him an interminable time.

The professor did not come out of the cave.

“Whew!” exclaimed the old sea captain, finally. “Dash my timbers, but I’m afraid he’s come to harm.”

The more the captain pondered over the matter the deeper became his alarm.

At length he decided to go in quest of his companion.

He entered the cave and turned its angle just as the professor had done.

Only a strange sense of intuition and a swift downward glance saved the captain’s life at that moment.

He saw a deep and yawning abyss at his feet.

For a moment he was overcome with grisly horror.

He saw how easy it was for any one to unwittingly walk into that death hole. The light on his helmet partly displaced the gloom.

But unless one looked down he would be sure to walk over the edge.

That poor Von Bulow had done this there was no manner of reason to doubt.

For a moment the captain stood transfixed. It was a terrible reality. What was to be done?

It was some time before his nerves were steady enough to enable him to advance to the verge and peer over.

But all down below was as black as Erebus.

Forgetting himself, the captain tried to shout down into the abyss, but no answer came back, of course.

Was Von Bulow forever lost?

Was he buried beneath that coral reef, never to be seen again by human eyes? It was terrible!

The captain’s brain began to work in devising some scheme for rescue, but it was in vain.

He leaned far over the verge.

Ha! was he dreaming, or was his eyesight true? Was not that a star of light far down there in the darkness?

He believed it was.

Doubtless it was the electric light upon Von Bulow’s helmet.

But it was visible only a brief moment.

Then it disappeared.

The captain leaned yet further over the verge.

Unfortunate move! Suddenly and without warning he lost his balance! Over the edge like a flash he went.

Down into the abyss he sank; but it was not like falling through air.

He alighted without any serious jar upon a bed of sand fully fifty feet below. He was at the bottom of the pit.

The helmet light made visible objects near at hand.

The captain recovered himself and looked about him.

He saw white walls of coral and long cavernous passages leading in all directions.

He was really in the heart of the coral reef. But he looked in vain for the professor.

Von Bulow was not in sight.

Was the professor dead? Had he become the victim of some submarine monster? The captain did not believe this.

He proceeded to examine critically the bed of sand upon which he rested.

There were the marks of footprints and the part impress of a man’s form. Von Bulow had fallen here.

But he had also arisen, for the footprints here led into one of the passages.

Filled with excitement, Bell proceeded to follow them. He was soon deep in the passage.

And as he pressed on he saw a flickering light in the far distance.

Suddenly the light ceased to move and remained stationary. Bell knew what it meant full well.

The professor had turned and saw the captain following him. He was waiting for him.

Quickly Bell overtook his colleague. The two divers fairly embraced in their joy.

“I thought you were lost,” cried Bell. “I gave you up for dead.”

“Then you fell into the same trap!”

“Yes.”

“My soul! How terrible our position is!”

“Yes; it is bad.”

“We must get out of here or die. Do you believe it possible to do so?”

Captain Bell shrugged his shoulders.

“We have only to try,” he said.

“You are right.”

“Shall we not follow this passage to the end? It may yet have an upward trend.”

“You are right.”

So they set forth down the passage under the coral reef.

It seemed ages that they wandered on. There seemed no end to the passage.

They were rapidly growing exhausted. At length Bell sank panting down upon a shelf of coral.

“My soul!” he gasped. “I fear we are forever lost!”

“Perhaps we had better return,” shouted Von Bulow. “We seem to be going deeper into the center of the earth.”

But Captain Bell shook his head.

“No,” he replied. “We cannot go back now. Our only hope is in going on.”

So they staggered on again.

But unobserved by them all the while the passage had been trending upward. As good fortune had it they had chosen the only safe and sure way out of the reef.

Suddenly a dazzling light shone forth far ahead.

“The Dart!” gasped Bell, joyfully. “We are saved!”

It was truly the submarine boat.

A few moments later they came out of the cavern, and were in plain view of the boat.

They saw that the cave from which they emerged was only one of many which they had passed in their way from the boat some hours before.

For they had been a long time absent from the Dart.

Indeed, so long that Frank had become greatly worried, and had even donned his diving suit preparatory to going out to search for them.

But just as the young inventor was about to go forth Barney cried:

“Dere they are, Misther Frank!”

Sure enough, the two divers were seen rapidly approaching the Dart.

“Mercy!” exclaimed Frank, with a deep breath. “I am thankful for that. I had given them up for lost.”

But even as he spoke he gave a great shout of alarm.

Behind the two men there suddenly appeared a giant form.

Frank saw that it was an octopus. Its long tentacles were ready to grasp them. It was a moment of fearful peril.

Barney rushed to the observation window, screaming and waving his arms wildly.

“Look out wid yez!” he shouted. “Shure, don’t yez see phwat’s behind yez?”

Of course the two men did not hear these words, but they saw Barney’s actions and at once understood.

They turned quickly, but it was too late.

Von Bulow was instantly encircled by a tentacle. Captain Bell was just quick enough to avoid one.

Frank Reade, Jr., saw that only the most desperate of action would save the scientist then.

He sprang down into the vestibule with an ax in his hand. He had already closed down his helmet. He closed the door and flooded the vestibule.