The submarine boat ran on until the lights of the schooner were lost below the horizon. When daylight came there was naught but a clear sea between them.

“We are in the waters of the Gulf of Mexico,” said Frank, with some satisfaction. “Cuba is on our lee.”

This caused all a thrill of excitement.

It was not a long run to the extremity of the “ever-faithful isle,” and then the Channel of Yucatan would be reached.

Frank reckoned that they would make it easily by the next noon. The Meta would be certainly a day longer covering the same distance.

That the schooner would really visit the locality Frank 21had every reason to believe. But he anticipated no trouble in keeping out of her way.

Steadily onward the Sea Diver ran. At length land was sighted to the southwest.

“The Isle of Cuba!” cried Wade.

“No,” said Frank; “Cuba is farther to the east. Those are some little cays which lay off the coast. We must bear off a little to the west.”

At this moment Barney sighted a sail. It was a coast lugger and carried some trading natives from Honduras.

They signaled the Diver in a careless sort of way and then scurried off to the eastward.

“They are afraid of us,” cried Wade.

Frank laughed.

“Doubtless they think us a government boat or war vessel,” he said. “Their trade may be of an illicit kind.”

However this was, the lugger certainly got out of the way very rapidly. She was soon lost to view.

They were now well into the Yucatan Channel. Wilbur Wade was a very busy man.

He was hurriedly making his reckoning and drawing his lines for exploration. The Sea Diver sailed slowly westward the while.

At length she came to a stop. It was at this point that the quest was to begin.

If there was really such a thing in existence as the Sunken Isthmus this was certainly the locality to look for it.

“I have been thinking about that sunken Isle of Mona,” said Wade. “Why might it not have been a part of the isthmus?”

“Very likely,” agreed Frank. “We may be directly over it now.”

“Eh!” exclaimed Wade, with a start. “And if we should come across the treasure——”

The two men looked at each other.

“I don’t see why it would not be lawfully ours as well as Poole’s,” said Frank. “Yet, I have no hankering for it.”

“Still, we might rescue it from the sea. Certainly if Poole gets it he will make no good use of it. It would be a mercy to cheat him out of it. Yet we have given him our oath——”

“Pshaw!” said Frank. “That does not count. It is not binding after what followed.”

“That is true. However, I don’t believe there is a very great chance of our finding any treasure. Let it stay down there with the drowned Caribs. Doubtless it is accursed gold.”

22“I am agreeable.”

Down settled the boat.

The electric lights flashed forth, and now a remarkable scene was presented to the view of the voyagers.

In that one moment of strained gaze it was seen that the hypothesis of the Sunken Isthmus was a thrilling truth.

It was an exciting fact.

Below them the searchlight showed a deep defile between rocky hills. And upon the sides of those hills there were leafless trees, with arms and branches and trunks as natural as life.

The wonderful chemical action of the water in these seas had doubtless caused a species of petrifaction. Astounded, the voyagers gazed upon the wonderful spectacle.

Wade was right in his element.

His hobby, his pet theory, had found verification. It only needed more extensive research to establish the fact of the complete isthmus.

For there was, of course, always the chance that this might be a part of the sunken Island of Mona. But Wade would not credit this.

“It is the isthmus,” he declared; “of that I am very sure. Now, to locate its coasts and contour. This can only be done by following it.”

So the Diver sailed slowly on, it being an easy matter to trace the line of the sunken shore.

“Which way shall we go first?” asked Frank. “To Cape San Antonio or to Cape Catoche?”

“To the last,” replied Wade; “then we’ll come back and make a sure thing of the lines.”

Over the deep defiles and rocky heights the submarine boat sailed slowly on.

The searchlight was kept at work, flashing hither and thither, and every new object of interest was carefully studied.

Soon the topography of the Sunken Isthmus began to change. The rocky hills sloped gradually away into a plain.

Here the Diver descended very close to the bed of the sea and Wade outlined quite distinctly the original coast.

The sinking of the earth’s crust, which had resulted in this submersion of the isthmus, was not to be easily explained. Some internal revolution was very likely responsible for it.

“Now,” cried Wade, after some careful study, “let us change our course to the south; I would like to know the exact width of the isthmus.”

The Diver’s course was changed accordingly. For four hours it sailed over the deep-sea plain. Then indications 23were plainly seen which told that this was the southern extremity of the isthmus.

“Fifty miles,” announced Frank, as he consulted the gauge; “that is the breadth of the isthmus at this point.”

“It is probably the average breadth,” said Wade, “although it is not impossible that it may have been wider in some other localities.”

The course was now changed to the east. It was not a great distance to the Cuban coast.

Half a day’s steady deep-sea sailing showed the usual signs of the surface, and Frank brought the Diver to a stop just over a jagged reef of coral.

There were great reaches of sand before them which trended upwards. That they ultimately rose above the surface in the form of a beach there was no doubt.

“We have reached the end of the isthmus,” declared Frank, “or at least that end which once joined the Cuban isle.”

“To make sure of it,” said Wade, “suppose we rise to the surface.”

Frank touched the tank-lever and the boat sprung upward. The next moment it was above the surface.

But all was darkness upon the sea. The hour was 4 A. M. and the sun had not yet colored the east.

But Frank turned on the searchlight and showed the cliffs distant not quite a half mile. That it was the Cuban isle there could be little doubt.

However, to make sure, the Diver lay-to off the coast until daybreak. Then bearings were taken. A small pearl-fishing sloop passed near.

Wade hailed it and learned for a fact that the coast was that of Cuba. Then he said:

“Let us go back to the deep sea. I ask only to follow the isthmus to the peninsula of Yucatan. All doubt will then be settled. My friend, Professor Brown, will then be very willing to admit his error.”

“He will if he is not pig-headed,” said Frank.

He was about to touch the tank-lever when Barney from the deck, gave a sudden sharp cry:

“Whurroo, Misther Frank, shure, it’s a sail off to windward!”

“A sail?”

Frank and Wade gazed in that direction. Then both gave a violent start. A small schooner was seen bearing down upon the Diver.

Wade’s eyes dilated.

“It is the Meta!” he exclaimed; “they are making for us!”

24This was the truth. Hardy Poole’s piratical schooner it was, and they had sighted the submarine boat.

She was bearing down rapidly, with all sails set. Frank and his companions watched the schooner with some curiosity.

But Frank knew that it would never do for the schooner to come within cannon shot. One ball striking the submarine boat would be likely to ruin her.

So he ran up a signal flag of defiance. It was seen by Poole, and a cannon was fired in reply.

Then the voyagers skipped into the cabin and Frank sent the Diver to the bottom.

Westward now over the sunken isthmus her prow was turned. For hours she kept on.

Toward night the place where they had first descended was reached. Here a stop was made.

Thus far the trip was a glowing success. No serious mishap had marred the project.

But could the submarine voyagers have read the future they would have experienced not a little of fear and dread apprehension.

Thrilling events were in store.

So far there had been discovered no indication or logical evidence of the possible existence of inhabitants on the isthmus in former days. Wade was not a little disappointed.

But when he remembered that there was yet a goodly distance between them and the Yucatan coast he did not altogether lose faith.

He hoped for the best.

All were somewhat exhausted with the incidents of the past forty-eight hours, so they were glad enough to turn in, Barney being left on guard, to be relieved by Pomp later in the night.

This was the usual arrangement.

Barney was completely exhausted himself, and in spite of his efforts to the contrary fell asleep at his post.

He was far advanced into the mystic Land of Nod when a startling thing happened.

From the gloom of a defile near there appeared a monster black form. It glided once or twice around the Diver as if to size it up.

The creature’s powerful curiosity was aroused and it ventured into the glare of the searchlight.

It was a strange creature.

Had Barney been awake he would have seen a monster specimen of a fish which seemed a cross between a shark and a whale. But he failed to see it at that moment, though he speedily became aware of its proximity.