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