It was true. The long sinuous strands of ocean grass, known under the name of "serpent weed," had caught around the whirling propellers and there had been wound and twisted very tightly. Just as sometimes the stern line gets so tightly twisted around a motor boat propeller as to require hours of work with an axe to free it, the seaweed was twisted around the blades of the M. N. 1.

Slowly the undersea craft came to a stop, and there she remained, floating freely enough, but a few feet above the bottom of the ocean. There was a look of alarm on the faces of Ned and Mr. Damon, but Tom Swift smiled.

"This is annoying, and may cause us delay," he announced, "but there is no danger."

"How are we to get free from the weed?" asked Mr. Damon. "We can't move if it's wound around our propellers, can we?"

"Not very well," Tom answered. "But all that will have to be done will be for some of us to put on diving suits, go out and chop the strands of weed away. We can do it more easily than could an ordinary vessel, for they would have to go into dry dock for the purpose. I think I'll go out myself. I want to look around a little."

"I'll go with you," said Ned. "As long as we haven't seen any sharks I don't mind."

"Nor gigantic starfish, either," added Tom with a smile, and Ned nodded in agreement.

"We might try reversing the propellers," suggested the man from the engine room, who had come in with the information about the serpent weed. "The chief didn't like to try that. We saw the weed from our observation windows and stopped as soon as we felt we had fouled it."

"That was right," commended Tom. "Well, try reversing. It can't do any harm, and it may make it easier for us to free the propellers when we go out."

He went to the engine room himself to see that everything was properly attended to. Slowly the motors were reversed, and only a slight current was given them, as, with the resistance of the tightly wound weed, too powerful a force might burn out the insulation.

Slowly the starting lever was thrown over. There was a low humming and whining as the current jumped from the batteries, and a slight vibration of the craft. Tom looked at the movable pointer which showed the speed and direction of the propellers. The hand oscillated slightly and then stopped.

"Shut off the current!" cried Tom. "It's of no use. The propellers are held as tight as a drum! We've got to go out and cut loose the serpent weed!"

The experiment of reversing the propellers had failed. But still Tom did not believe his craft was in danger. He gave orders for the engine room force to stand by and then arranged for himself, Ned, and Koku to go outside in diving dress and cut the weed off the shafts. There were twin propellers on the submarine, each revolving independently by separate motors, and each capable of being sent in forward or reverse direction.

"Start the engines as soon as we give the signal," Tom told the machinist. "Two knocks on the hull with an axe will mean go ahead, and three will mean reverse."

"I understand," said Weyth, the machinist. "But stand away from the propellers after you give the signal. I'll give you three minutes to move clear."

"That will be enough," Tom said. "But better make it half speed in either case. My idea is that if we can partly cut the weed off, starting the propellers, either forward or in reverse, will finish the trick."

"It may," agreed Weyth.

Armed with axes and sharp steel bars, Tom, Ned, and Koku were soon ready to step outside the submarine.

They entered the diving chamber. In the usual manner water was admitted, and, when the pressure was equalized, the outer door was opened and they walked out on the floor of the ocean, the submarine having been allowed to settle down again on the bottom of the Atlantic.

The powerful searchlight had been turned so that the beams were diffused toward the stern. In addition to this Tom and his two companions carried, attached to their suits, small, but brilliant, electric torches. Of course they had their air tanks with them, and also the telephones, by means of which they could communicate with one another.

As they emerged into the warm waters surrounding the submarine they disturbed thousands of small fish which were feeding all about. Like ocean swallows, the creatures scattered in all directions, some even brushing the divers as they slowly made their way toward the stern of the craft.

"Nice place here," said Ned to Tom, as they walked along, Koku coming just behind them.

"Yes. If we could take this up above and exhibit it in some city park it would make a hit all right," answered the young inventor.

They were walking on the pure, white, sandy floor of the ocean, some seven hundred feet below the surface, protected from the awful pressure of the water by means of the specially constructed suits which Tom had invented. About them, growing as if in a garden, were great masses of coral, some so thin and sinuous that it waved as do palms and ferns in the open air. Other coral was in great rock masses.

Then, too, there was the unpleasant serpent weed. It did not grow all over, but in patches here and there, as rank grass springs up in a meadow.

And it had been the misfortune of the M. N. 1 that she poked her tail into a mass of this long, tough grass, which was now wound about her propellers.

In addition to the many wonderful vegetable forms that grew on the ocean floor, some rivalling in beauty the orchids of the tropics, and almost as delicate, there were the fishes, which darted to and fro, now swiftly swimming beneath some coral arch, and again poising around some mass of waving sea fronds.

"Well, let's get busy," called Tom to Ned through the telephone. "We want to free the propellers and find the wreck of the Pandora. She may be a hundred feet from us, or a mile away, and in that case it's going to take longer to locate her."

Together they walked to the stern of the disabled craft. One look at the propeller shafts, the examination being made by the diffused glow from the searchlight, as well as from the electric torches carried, showed that the diagnosis of the trouble was correct.

Wound around both propellers was a mass of the serpent weed, tightly bound because the machinery had whirled it around and around after the grass had once been caught. It was almost as bad as though manila cable had been thus accidentally fastened.

"Well, might as well begin to cut it loose," said Tom to his companions. "Koku, you take the port propeller, and Ned and I will work on the other. You ought to be able to beat us at this game."

"Me do," said the giant, as he got his axe ready for work.

Blows struck in water lose much of their force. This can easily be proved by filling a bathtub full of water, rolling up the sleeves, and then taking a hammer in the hand, immersing it fully, and trying to strike some object held in the other hand. The water hampers the blows.

It was this way with Tom and his friends. Nearly half of Koku's great strength was wasted. But they knew they could take their time, though they did not want to waste many hours.

The streamers of weed were like strands of tightly wound rope, and this, under certain circumstances, acquires almost the density of wood. Tom and Ned, working together, had managed to chop a little off their propeller shaft, and Koku had done somewhat better with his task, when Ned became aware of a shadow passing above him.

Instinctively he looked up, and as he did so he could not repress a start of horror. Tom, too, as well as Koku, saw the menacing shadow. Ned grasped more tightly his sharp, steel bar and spoke through the telephone to his companions.

"Devil fish!" he said. "The devil fish are after us."