Born in 1601, Athanasius Kircher has been hailed as the "last Renaissance Man" owing to his scholarly works in fields as diverse as biology, geology, medicine and technology. Among his most remarkable books was Mundus Subterraneus, a study of the Earth's interior that might have inspired Jules Verne.

Kircher wrote his 1664 opus some three decades after he undertook an expedition inside Mt. Vesuvius — around the time it had experienced its first major eruption in centuries. The interior of the volcano, he wrote, was, "all up and down everywhere, cragged and broken, while its chamber was "made hollow directly and straight." The bottom of the crater was"boiling with an everlasting gushing forth, and streamings of smoke and flames, and employed in decocting Sulphur, Bitumen and the melting and burning of other kinds of Minerals."

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As The Public Domain Review notes:

It was within this hollow mountain that Kircher really began to develop the theories he set down so many years later in Mundus Subterraneus, to envision what it might be like even deeper within the earth, and how the mountains and fires and rivers and oceans might somehow all be connected.

He sketched two diagrams, depicting how fire (image above) and water (below) were flowed within the Earth and on its surface.

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"The whole Earth is not solid but everywhere gaping, and hollowed with empty rooms and spaces, and hidden burrows." he wrote, explaining that, deep below, it holds many great oceans and fires, interacting with one another through passageways that reached all the way to the planet's core. In Kircher's view, volcanoes, though awe-inspiring, were "nothing but the vent-holes, or breath-pipes of Nature," while earthquakes were the "proper effects of sub-terrestrial cumbustions."

Kircher added that "the fire and water sweetly conspire together in mutual service." The lunar tides push "an immense bulk of water" through "hidden and occult passages at the bottom of the Ocean... into the intimate bowels of the Earth." The oceans, which would freeze without the fires, also kept the fires in check to prevent "unlimited eruptions." The "secret make-up of the mountains" is that they are hollow and serve as reservoirs. Hot springs and fountains, he believed, are produced where underground water passageways intersect with the fire channels.

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The Public Domain Review says that Mundus Subterraneus had at least two direct influences on popular culture:

The creation of Bernini's Fountain of the Four Rivers in Rome was certainly informed by Kircher's ideas about the interior structure of the earth as well as his ideas about the mystical nature of the universe (that's another story). And the spirited, bumbling polymath-dreamer of Jules Verne's A Journey to the Center of the Earth, who leads a subterranean descent through an old volcanic crater, certainly seems to be based on Kircher himself.