> According to a 1716 paper mentioned in the book, Hollow Earth: The Long and Curious History of Imagining Strange Lands, Halley devised the Hollow Earth theory as a way to explain strange disturbances in the magnetic field that were connected to the aurora borealis — Earth’s natural light show. Halley was right about the magnetic field, but not so much about Hollow Earth..

> Years later, War of 1812 veteran and explorer, John Cleves Symmes, seized on the Hollow Earth theory.. to justify American expansionism.. into Antarctica. Similar to Halley, Symmes believed that the Earth was made of a hollow shell 800 miles thick, but he added his own twist: there were openings to Earth’s hollow core at both the North and South Pole. These he called “Symmes holes.” There, he believed that light — and human explorers — could descend underneath the surface into the hollow core.

There was also a German sci-fi novel "Two Planets" by Kurd Lasswitz (1897), which influenced a young Werner von Braun who would later contribute to German rocketry and American space program, and write his own sci-fi novel about Mars exploration, https://news.ycombinator.com/item?id=39571588

The park is presented as a lovely old commune with vintage bungalows, a rare place where women actually ran things in the late 1800's and had a lot of freedom within the community to create art, plays, and raise their kids. A lot of the women were widows and this was basically the only option for them aside from marrying their dead husband's brother, or some other arcane ritual of the day. They paid for the upkeep and food for the "commune" by staging plays, and the men dealt with the outside world and negotiated for such transactions (to keep up external appearances of a patriarchy, something that was totally expected back then.)

My wife and I were touring the place, having a good old time, and when we entered the community gathering hall/church, my wife started slapping my arm repeatedly to get my attention. She's gasping for air in an almost shocked laughter. She pointed to the giant model of the split-open Earth as interior, like the one shown at the top this article. The whole commune were hollow Earthers!!

Of course, the bloom was then very much off the rose. A few things to know about Teed, in addition to the hollow Earth stuff, which we subsequently learned by reading more deeply into the interpretive museum text, therein:

1: Teed believed he was Jesus Christ, or at least, that he would come back with Christ. When he died, the cult left him on the porch for a few weeks, waiting for him to get back up. The Sheriff had to come and demand they bury him, because the stink was so bad from the rotting corpse in the hot Florida sun. A testament to the power of belief, or delusion, you decide.

2: The last Koreshanty-ist (As they were called) born and living on the Florida property admitted she was wrong about the Earth when she watched the moon landing on TV in 1969 in her bungalow.

3: Teed believed his theory was true because he said if you walked in a flat, straight line in any direction, you would eventually hit an incline. This was absolutely true! BECAUSE THEY LIVED IN A SWAMP.

4: Of course, 100% as expected, despite the ladies running the cult via committee and being very free in the cult society... All men except Teed were to remain celibate, but--OMG!-- there were plenty of babies! Teed 100% did the cult leader thing and had relations with every woman on the property. That's the real reason he built the cult.

Like, that's also a pretty ridiculous scenario of planetogenesis, but it's at least somewhat plausible.

So, as a Friday afternoon thought, one can hypothesize the existence of large-ish (~hundreds of meters) empty cavities within the mantle with some simple animal life (worms, insects?), which doesn't sound totally off the rails to me. I wonder if our current seismic analysis technology can detect such cavities (chatGPT says waves to propel core have wavelengths on the order of kilometers).