"It's some sort of land," he announced. "We'll go over in the morning and see what it is. Probably it's an island, for there's no main land in these parts. We are in the middle of the southern Atlantic now." The next morning, after breakfast, the _Porpoise_ was headed toward the dark spot on the surface of the water that the professor had gazed at the evening before. As they came nearer it was seen that the place was a large island. "But it's a mighty queer one," spoke Mr. Henderson. "It looks more like a big volcano than anything else." As the ship came nearer it was seen that this was true. The island rose abruptly from the surface of the sea in a big ridge, slightly rounded. There appeared to be no signs of life on the land, but in the air overhead hovered several big birds. These circled about and then fluttered down, seemingly about the middle of the island. "We'll sail around and see if there's a place to land," spoke the inventor. "There doesn't seem to be a good harbor on this side." Slowly the _Porpoise_ made the circuit. The island appeared to be almost round. When they had gone about half way around Andy, who was staring ahead, cried out: "Look out Professor! Don't go any nearer or we'll be sucked into the whirlpool!" The inventor looked where the hunter pointed. Then he beheld the strangest sight he had ever seen. The island was low toward where Andy pointed and they beheld the waters of the ocean pouring over the edge of it, and falling down into an immense hole with a roar like that of Niagara Falls. "Reverse the ship!" cried Professor Henderson. "Send her back quickly, Washington, or we'll be sucked down!" The colored man lost no time, and the big screw was sent whirling in the opposite direction. And it was high time, for already the onward rush of the falling waters was slowly drawing the ship toward the big cavern. "That was a lucky escape," commented Amos Henderson. "Well, as we can't land there we'll try the other way around." The ship was headed in the opposite direction, and, after an hour's sailing, a good harbor was discovered. The _Porpoise_ was anchored in shallow water close to the shore and in a small boat the professor, Andy and the two boys went to the strange island. They found it merely an immense circle of land with the middle part taken up by the big hole. And such a hole as it was! It was so wide across that they could not see the farther side, and the depth they could only guess at. Looking down they could only see great rolling masses of clouds or vapor. "Perhaps it's steam," suggested Jack. "Maybe it is," agreed the professor. "If this is a volcano, with lava in it, the water of the ocean, pouring in on the other side, may be changed to steam." "Do you suppose this hole leads to the centre of the earth?" asked Mark. "I've read somewhere, that the earth is hollow."