Frank had recovered quickly and emerged from his place of safety just in time to see the sea serpent strike the Dart.

The young inventor had seen and realized the awful risk which this entailed, and muttered:

“My goodness! We are all lost!”

But the result of the serpent’s attack was indeed gratifying.

And he also saw what he believed to be his opportunity. Drawing his knife he darted after the monster.

It was lying half dormant on the floor of the cavern from the shock which it had received.

But as Frank ran toward the Dart he saw Barney coming toward him.

Barney fairly embraced his young master, as he cried, placing his helmet close to Frank’s:

“Och hone, Misther Frank, an’ I thought it was kilt entoirely ye was!”

“I had a close call,” replied Frank. “But where is the captain?”

“Shure, he’s safe aboard, sor.”

“Good! Now, Barney, we’ve got to kill that monster some way.”

The Celt looked at the dormant serpent a moment, and then swung his ax aloft, saying:

“Shure, an’ it’s wid yez I am, Misther Frank. Say the worrud an’ I’ll go up on this side of him an’ cut his head off.”

“Let me take your ax,” said Frank, resolutely.

Barney complied and drew his knife. Frank made a motion for him to follow.

The serpent was quickly recovering from his stupor.

Frank saw that there was no time to lose, and at once made a bold attack. When near the monster’s head he rushed forward.

The serpent reared its horrible jaws and seemed about to strike Frank; but the young inventor struck first.

The keen blade of the ax swung around and took the serpent full in the jaw.

It was a telling blow.

It fairly sliced away a portion of the monster’s jaw and filled the water with blood. Again Frank swung the ax aloft.

Barney attacked the body of the serpent, trying to cut the huge coil in two.

The attack was a success.

Again Frank’s ax struck the serpent full in the neck, cutting a huge gash.

Then the maddened reptile made a savage blow at Frank.

It just missed him by a narrow margin and proved the end of the struggle.

Frank saw his opportunity, and gave the reptile a blow which almost severed its head from its body.

The monster’s huge coils went writhing and twisting into the depths of the cavern.

The struggle was over.

Frank and Barney, somewhat exhausted by the struggle, climbed aboard the Dart.

They were joyfully welcomed by the others, and mutual congratulations were exchanged over the success of the fight.

“Begorra, I thought shure it was the ind av Misther Frank!” cried Barney. “Shure, it wud have been a sorry day for the loikes av us!”

“Golly, if I had jes’ been out dere I would hab been happy!” declared Pomp. “I was jes’ itching fo’ to git a crack at dat ar big rapscallion of a snake.”

“Well, as for me,” said Von Bulow, with a laugh, “I quite distinguished myself by running away. But I was never cut out for a fighting man anyway.”

“And I stayed at home,” rejoined Bell. “Frank, you and Barney are the heroes.”

All were intensely hungry, and Pomp served up a steaming repast.

There was lovely steak from the swordfish, crabs on toast, fresh and nice, and many other saline delicacies, which were easily procured in the sea.

The explorers regaled themselves sumptuously, and then all turned in for a sleep.

Frank had decided to spend some hours longer in the cavern.

When they awoke six hours later, Frank went into the pilot-house and started the Dart for the mouth of the immense ocean cavern.

In due time this was reached, and soon they were not so very far from the spot where Captain Bell’s treasure ship had sunk.

All were now eagerly on the lookout for the wreck.

The searchlight’s rays were sent in every direction through the ocean depths.

Suddenly Captain Bell, who was forward on the lookout, shouted:

“Wreck ahoy!”

The announcement went through the boat with startling force.

Everybody was at once on the qui vive.

And now dead ahead was seen a huge black mass looming up through the water. It was a sunken ship.

Of course all believed it to be the Vestal Virgin.

But the wreck was so covered with silt and seaweed that its character could not well be identified.

The submarine boat sailed around it twice, then Frank allowed it to come to a rest on the ocean floor of white sand.

“What do you make of it, skipper?” asked Captain Bell, as Frank came out of the pilot-house.

“I hardly know,” replied Frank. “It looks to me, though, like a ship of more modern build than the pirate vessel.”

“It’s mighty hard to tell for the seaweed over it.”

“Yes.”

“But I think it’s the Virgin!”

“You do?”

“Yes; she’s in about the right location. It must be her.”

“I hope so.”

Preparations were now made to go out and inspect the submarine wreck. This fell to the lot of Frank, Von Bulow and the captain.

Barney and Pomp remained behind.

They were very quickly equipped for the expedition; armed with axes and saws and such tools as were deemed necessary, they left the Dart.

It was an easy matter to climb over the kelp-strewn rocks until the sunken vessel was reached.

It lay half upon its side, and its port rail was nearly on a level with a drift of hard, white sand.

This made it an easy matter for the explorers to reach the deck.

They simply walked up the sandy slope and climbed over the rail.

In the glare of the electric light, the deck was seen to be in a state of wild disorder.

Rotting spars and heaps of debris covered it from stem to stern.

It was easy to see that the vessel had passed through a terrible experience at sea.

The storm which sent it to the bottom must have been a fearful one.

It required no further examination to satisfy the party that this was not the treasure ship.

Captain Bell saw at once that it was not the Vestal Virgin, and putting his helmet close to Frank’s, shouted:

“This is not the ship.”

“It looks like a merchantman,” replied Frank.

“It is.”

“Moreover, it was never sent to the bottom by shotted guns. It went down in a fearful storm.”

“Without a doubt. But the Virgin must have gone down in this vicinity.”

“Yes.”

“We will probably find her not far from here.”

“Well,” said Frank, doubtfully, “is it worth while to explore this hulk? She probably did not carry money.”

Von Bulow, however, was in favor of exploring the sunken merchantman.

“For curiosity, if nothing else,” he explained. “I’m quite anxious.”

“Very well,” agreed Frank. “It shall be so.”

With which the young inventor crossed the deck. He reached the companionway which led into the cabin.

This was closed, but a blow with an ax forced it in.

The stairs that led downward into the cabin were crumbling with decay.

Frank led the way down.

The light upon his helmet was sufficiently bright to reveal objects below quite plainly.

Von Bulow and the captain followed. All stood at the foot of the companion ladder.

The cabin was in a fearful state of dissolution.

The elegant furnishings were all rotten and in shreds, and even the cabin table was shredded by sea worms.

But the explorers did not pause here long.

They passed through and into the forward cabin. Here was the long mess table, and upon it were dishes and eating utensils, just as the men had been served, which was the last ever eaten on board the ship.

Frank took up one of the plates. In the china was the imperishable mark usually placed upon all ships’ ware with the name:

“Ship Tempest, Baltimore.”

This was all that could be learned of the identity of the vessel or of its mission. Yet it was reasonable to suppose that she was a merchantman.

Little more of interest was found aboard her.

A few skeletons of the members of the crew and some corroded coins. This was all of value.

The party retraced their steps to the deck. Frank was the first to spring up out of the companionway, and as he did so he was given a startling shock.

Until now the wreck had been flooded with a brilliant light from the searchlight of the Dart.

But this was no longer so.

All was the darkness of the ocean depths about. Nothing could be seen beyond the slight radius made by the light on their helmets.

The Dart had left them.

What did it mean?

For a moment the explorers were appalled with the most startling realization.

Left at the bottom of the ocean, upon a sunken wreck.

There was no possible way of ever reaching the surface.

That is unless the Dart should return from where it had gone, and why it should have left them in this manner was a mystery.

Frank knew that Barney and Pomp would not leave the vicinity for any light reason.

“Something has happened!” he exclaimed in dismay.

“The Dart has met with a mishap.”

“My goodness!” exclaimed Von Bulow; “then we are lost!”

“What could have happened?” asked Bell in horror.

Their three helmets were close together at this moment. The only logical conclusion that Frank could arrive at was that the Dart had received some fearful shock and had gone to the surface.

If this was the case it would perhaps shortly return.

But the one horrifying thought which oppressed Frank was that possibly Barney and Pomp would lose their bearings and would not be able to find the three divers.