Gardner's theory postulates that the earth is not a solid sphere but rather a hollow shell with a thick, dense layer at the outer surface. He argues that there are large openings at the North and South poles that lead into the interior of the earth.The book describes a journey taken by a fictional character named Olaf Jansen, who claims to have sailed through one of these openings and journeyed to the earth's interior. There, he encounters a hidden world with its own sun and inhabited by a highly advanced civilization.Gardner's theory was based on a number of scientific observations, including the fact that the earth's magnetic field behaves as if there were large openings at the poles. However, it has been widely discredited by modern scientific understanding, which holds that the earth is a solid, dense sphere.Despite its lack of scientific credibility, "A Journey to the Earth's Interior" remains a fascinating work of speculative fiction that continues to capture the imagination of readers interested in alternative theories of the earth's structure and history.