But the needles didn't point north...?

It was the seventeenth century, the Age of Enlightenment and, from ports around Europe, flotillas of ships set sail to cross the oceans, to explore and trade around the world. Aboard these ships, mariners of the time tracked their voyages, navigating the open seas with the aid of the sun and stars. Tracking their positions, north or south, was straightforward. It required only an observation of the sun or the North Star, but meticulous tables and charts tracking the stars were necessary for navigators to determine their position, to the east or west. In time, precise marine chronometers, capable of maintaining an accurate time through wind and wave, would become available. This would simplify the process required for navigators to determine their east or west position.

One of the tools which navigators of the Age of Enlightenment did possess was a simple magnetic needle, balanced so as to be able to spin freely. The needle or "compass" would align itself with the magnetic field of the Earth, pointing to the magnet poles, and giving navigators an indication of which way was north, even when rain or fog might obscure other indicators. But as navigators used and reused compasses over centuries they noticed that compasses did not always point north. Instead, they deviated, varying in unpredictable ways that could lead ships astray.

Royal Society member and noted astronomer, Edmund Halley (yes, the one with the famous comet named for him) pondered why compasses might vary. Over a number of years he acquired data on magnetic deviations in many different places and over a period of time. From this data, Halley determined that poles were moving in a gradual, but regular motion. But what might this mean?

In 1681, in a series of papers that Halley read to his colleagues in the Royal Society, he proposed a radical idea. The Earth, Halley suggested, was hollow, consisting of an outer shell around an inner sphere. This inner sphere had its own magnetic poles and gaps between it and the outer shell left it free to move. This movement of this inner sphere caused the variation in the magnetic fields and poles. Halley would later expand his theories to include a total of three concentric spheres, one inside another. He would also suggest that these inner spheres might also harbor life of some kind.

Halley's theories, as outlandish as they may seem today, do have a resemblance to our understanding of the geology of the Earth. Rather than concentric, hollow spheres, the Earth has spherical layers -- but one of them is largely comprised of molten iron. Movements within this liquid layer cause electrical currents and a magnetic field that drifts over time.

Even before the time of Edmund Halley, stories of civilizations existing in hollows deep underneath the ground existed. In the nineteenth century, would be explorers proposed expeditions to the north and south poles, arguing that these remote places, some of the last unexplored areas on Earth's surface, might contain passages to hollows within the center of the Earth. An entire genre of literature grew up, telling stories of explorers heading to these locations, and finding lost utopian civilizations deep into the center of the Earth. People even posited that the entire planet is hollow and we are living on the inner surface, rather than the outer. Even today, the idea of a hollow Earth continues to intrigue and captivate those looking for one more adventure into an uncharted land.

In 1997, former Library of Congress Bibliographer and Senior Science Specialist, Ruth S. Freitag, published a list of references covering hollow earth theory and its advocates. This guide is both a reproduction and a continuation of that original guide, introducing more recently produced material while preserving the content of Freitag's original work, and adding links to the catalog and online versions where available.