He moved his machine shop to a lonely spot on the Maine coast, and there, with the help of the boys, Washington, Andy and two machinists constructed a submarine boat, called the _Porpoise_. In this the professor resolved to seek the south pole, he having a theory that it was surrounded by an open sea. After much hard work the _Porpoise_ was made ready for the voyage. What occurred on this great trip is described in the second book of this series, called “Under the Ocean to the South Pole, or the Strange Cruise of the Submarine Wonder.” In that is told how once more Tom and Bill, with Andy, the boys and Washington, accompanying Professor Henderson, had many thrilling experiences. They were caught in the grip of the grass of the terrible Sargasso Sea. Monstrous suckers grasped the boat in their powerful arms, and had to be fought off. They were caught in a sea of boiling water and imprisoned between big fields of ice. By means of strong diving suits they were able to leave the ship and walk about on the bottom of the sea. They visited a graveyard of sunken ships, saw many strange monsters as well as many beautiful fish in the great depths to which they sunk. Many times they were in dire peril but the resources of the professor, the bravery and daring of the boys, no less than the help Washington and Andy Sudds, the hunter, rendered at times, brought them through. Those of you who read of their adventures will recall the strange island which they came upon in the Atlantic Ocean, far from the coast of South America. When they first drew near this island they were almost sucked into the depths of a great whirlpool, caused by water pouring down a big hole that seemed to lead far into the earth. They reversed their ship just in time. But, on going to another side of the island they were able to approach safely, as at this point the great hole was farther from the shore. Then they landed and investigated. They found the island was almost circular, and the hole was also round, but not in the center of the land. It was an immense cavity, so wide they could not see across, and as for the depth they could only guess at it. Looking down they could only see rolling masses of vapor and clouds caused by the water which poured down from the ocean with the force of a Niagara. Gazing down into the big hole Mark suggested it might lead to the centre of the earth, which some scientists claim is hollow. The professor admitted that the cavity looked as though it led to China. They had no means of investigating further the mystery of the opening and returned to their submarine, completing the voyage to the south pole.