On their journey to the South Pole, the adventurers came upon a strange island in the Atlantic, far from the coast of South America. On it was a great whirlpool, into which the Porpoise was nearly sucked by a powerful current. They managed to escape, and had a glimpse of unfathomable depths. They passed on, but could not forget the strange hole in the island. Mark suggested that it might lead to the center of the earth, which is hollow, according to some scientists, and after some consideration, Professor Henderson, on his return from the South Pole, decided to go down the immense shaft. To do this required a different kind of vessel from any he had yet built. He would need one that could sail on the water, and yet float in the air like a balloon or aeroplane. How he built this queer craft and took a most remarkable voyage, you will find set down in the third book of this series, entitled "Five Thousand Miles Underground." In their new craft, called the Flying Mermaid, the professor, the boys, Washington and Andy, sailed until they came to the great shaft leading downward. Then the ship rose in the air and descended through clouds of vapor. After many perils they reached the center of the earth, where they found a strange race of beings. One day, to their horror, an earthquake dosed the shaft by which they had come to the center of the earth. The boys were in despair of ever getting to the surface again, but the professor had been prepared for this emergency, and he had built a strong cylinder, into which all the travelers placed themselves. Then it was projected into a powerful upward shooting column of water, which Professor Henderson hoped would take them to the surface of the earth. Nor was he mistaken. They had a terrible journey, but came safely out of it. They opened the cylinder, to find themselves floating on the sea, and they were rescued by a passing vessel. Of course, they had abandoned the Mermaid, leaving the craft in the center of the earth, but they had brought back with them some valuable diamonds, which formed their fortune. This ended, for a time, the experiments of the professor, who decided to settle down to a quiet life, and write out the observations he had made on the three voyages. The boys wanted to get an education, and, investing their share from the sale of the diamonds, they took up a course at the Universal Electrical and Chemical College. Each had an ambition to become as great an inventor as was Professor Henderson, with whom they continued to live in a small city on the Maine coast. Washington White and Andy Sudds also dwelt with the professor, Andy going off on occasional hunting trips, and Washington acting as a sort of body servant to Mr. Henderson. Jack and Mark had completed one term at the college, and were in the midst of the second when this story opens.