Washington set the big screw to revolving in the opposite direction. The _Porpoise_ shook and shivered but the mud held her fast.

"We must have gone in pretty deep," commented Amos Henderson. "Luckily it was soft mud instead of a rocky reef or we'd have damaged the ship beyond repair."

For several minutes the engines were kept on the reverse, but all to no purpose. The sticky mud was like glue in its holding power and the ship had buried her prow deep into it.

"I guess we'll have to dig our way out," said the professor, after taking a careful view of the big mud bank from the conning tower.

"Washington, get out the diving suits and the spades."

"Are we really going out in the water to dig?" asked Tom.

"Of course," said Mr. Henderson. "You'll be as safe as in the ship, wearing one of my diving suits. We'll all have to help, for it will be quite a task."

The queer suits were brought out, and the reservoirs in the helmets were filled with compressed air. It was decided to have Washington remain within the _Porpoise_, to watch the machinery and start the engine when the digging was partly done, in order to see if the ship would not pull herself free when some of the mud had been removed from the prow.

"And we may need Washington for another purpose," said the professor.

"What for?" asked Mark.

"Well, he'll have to stay by the diving tank, to let us in quickly in case of emergency."

"Do you think there'll be an emergency?" asked Jack.

"You never can tell," was the answer. "We are in deep water, and I don't want any accidents to happen."

In a few minutes all save Washington were in their diving suits and ready to go out and walk on the bottom of the sea. They entered the tank, the door was closed, and then water was slowly admitted from the ocean. When the tank was full, led by the professor, they stepped out on the muddy floor of the ocean.

At first the pressure of the water at so great a depth bothered them.

But, as we know, the diving suits were reinforced with plates of steel, and so strong that little more than an extra weight of water was noticed. They soon became used to it. Each one carried a spade, while the professor, Andy and the boys each had, slung about their necks by straps, one of the electric guns.

Cautiously they walked toward the big mud bank. They had to go slowly because of the weight of the water above them, and because they might at any minute step into some muck hole and sink down. Fortunately, however, they found there was a firm bottom right up to where the bank of mud reared upward.

Turning on the electric lights in their helmets, the voyagers were able to see quite distinctly. The _Porpoise_ had rammed her nose into the under-water hill for a distance of about ten feet. It was going to be no easy matter to get her free, but the divers lost no time.

Vigorously they attacked the big hill of mud. They dug their spades in and tossed the earth to one side. It was a strange place to work. At first the weight of water hampered every one, but they soon became used to it and were able to proceed more rapidly.

From the conning tower Washington kept watch of their progress. When they had gone in about five feet he started the engines, hoping the _Porpoise_ might now pull herself free. But the mud still held.

By signs, for it was of course impossible for any one to hear or speak, attired as they were in helmets and suits, the professor motioned that they must dig deeper.

Once more they attacked the big mud bank with their shovels. Farther and farther they went into the muck until it seemed that the nose of the submarine must be free. But when Washington started the engines it was obvious that the ship was still held.

Again the digging was resumed. All at once, while every one was wielding his spade to best advantage, a shadow seemed to cover the water. It loomed up large and black, and the professor stopped and gazed upward.

What he saw made him drop his spade and grab the gun that was about his neck.

Floating in the water above the diggers, were three immense sharks.

Their cruel mouths were partly open, showing three rows of big teeth, and they were slowly turning over on their backs to make a sudden rush and devour the men and boys. Owing to the peculiar shape of its maw a shark can not bite until it turns over.

The professor motioned for Bill and Tom to move behind him, and signalled for Jack, Mark and Andy to stand close with their weapons ready.

The sharks floated lazily downward as if they knew they had the diggers at their mercy. To run and escape was impossible, for no one could run hampered by the weight of water and his diving suit.

One of the terrible fish opened its mouth wider and, with a flirt of its tail aimed straight for the professor. Mr. Henderson raised his gun, and took careful aim at the middle of the fish, half turned over. Unerringly the electric bullet sped on its way. It entered the soft under part of the shark, and immediately the thing struggled in its death agony.

The water was dyed with blood. At the same instant the other sharks rushed forward in a swirl of foam. The boys and Andy fired as best they could, and must have hit one of the creatures for there was a greater commotion.

But the fight was not over. Instantly the ocean seemed alive with the giant fish. Attracted by the blood of the killed ones, scores of the tigers of the seas rushed toward the scene of combat making matters livelier than ever.

The professor, the boys and Andy fired their guns at random. Redder and more red became the water until their helmet lamps barely glowed in the crimson sea. It seemed that a whole army of the voracious sharks had attacked them.

The professor realized that to stay and attempt to fight all the sharks in that part of the ocean was impossible. He motioned for the boys and Andy to follow him. Then he slowly led the way back to the ship.

But the sharks were not to be gotten rid of so easily. Several of the largest followed the diggers, their horrible eyes, and big mouths with rows of cruel teeth, striking terror to the hearts of all.

One of the creatures made a rush for Bill and Tom who were close together. Either or both of the men would have been bitten in twain, in spite of the protection of their diving suits, had not Mark, with a snap shot, killed the fierce fish.

It was now a running fight, and yet not so much that, because to run was impossible. However, they hurried all they could, and, by dint of quick firing kept the ugly creatures at bay until the side of the ship, where the diving tank was placed, was reached.

The professor stepped to one side, and motioned for the others to proceed him in entering. Little time was lost. As Bill, the last one in, stepped past the steel door the inventor attempted to enter. To do so he had to let go of his gun.

Instantly one of the sharks made a rush for the old man. But Andy was on the watch. He leaned forward, and, from his weapon sent a bullet straight down the throat of the monster. The electric missile did its work well, and the lifeless body of the shark was devoured by the others of its tribe.

The professor pulled the door shut behind him. Then he set the pump to work to empty the tank. As he did so there was a tremor to the ship.

What could it mean?

In a few minutes the tank was empty and the divers stepped out into the ship, freed from the oppressive weight of water. The ship continued to vibrate and seemed to be in motion.

"What is it?" asked Mark as soon as he had his helmet off.

"De ship am free! We've got off de mud bank!" exclaimed Washington, running in from the engine room.

It was true. Enough had been dug so that, with the power of the screw working backward, there was sufficient force to pull the _Porpoise_ from her perilous position.

"Empty the tanks and rise to the surface," said the professor.

In a few minutes the ship was on top of the waves, the adventurers freed from the double danger of the mud and sharks. They congratulated each other on the good outcome of the fight with the monster fishes.

Wearied with their labors and the battle under the waves, the travelers sat up on the deck breathing in the fresh breeze. Then, after a while, supper was made ready and eaten with good appet.i.tes.

It was decided hereafter to sail along near the surface at night, and not to submerge the ship deeply save during daylight, when it was easier to distinguish objects under the water.

Following this plan the _Porpoise_ steamed along just awash that night, and the next day was sent down about fifty feet below the surface.

One afternoon, when the travelers were resting, having partaken of a fine meal, the professor went on deck to make some observations, the ship having been raised for that purpose. He came down, somewhat excited.

"Well, we're half way to the pole," he announced.

"How can you tell?" asked Mark.