Friday, November 21, 2025

Building Your Campaign Setting With The Hollow Earth Hypothesis

If you play RPGs, then chances are the idea of a Hollow Earth is probably not new to you. With games like Hollow Earth Expedition, Adventure To Hollow Earth for Mage: The Ascension, or Terra Incognita, a Hollow Earth setting for Dungeons and Dragons 5th Edition, the concept has been explored in a lot of ways. And that's without discussing Hollow Earth in popular fiction, such as the prehistoric Pellucidar.

However, just because an idea is popular, that doesn't mean everyone has heard of it. And if you haven't read the history of the Hollow Earth theory, it's pretty damn wild!

So, this is the Earth...

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The Origin of The Hollow Earth


According to the Library of Congress, the Hollow Earth theory was originally created back in the 1600s as a radical way of explaining a natural phenomenon. You see, when sailors would go out on their ships to cross the ocean, they would bring all the tools they had with them to make sure they didn't get lost. Star charts were common, as well as tools that helped them easily locate true north so they could orient themselves accordingly. However, it was noted that sometimes the magnetic needles in their compasses did not point north. They would, in fact, go wandering and become completely useless at certain points.

How could this happen? Well Royal Society member and astronomer Edmund Halley (yeah, the guy whose name is on the comet) had an idea... the Earth was actually hollow!

All right, I'm gonna back up. Tell me where I lost you.

So, the basic idea that Halley proposed was that the Earth we know up on the surface is just an outer shell, and there is actually an inner Earth with its own magnetic poles, and that these two Earths spin freely. This change in relative positioning of the inner Earth interfered with the natural magnetic laws on the surface, which is why the compass needles would sometimes wander for no discernible reason. He even went so far as to suggest that this inner sphere would harbor some kind of life.

On the one hand, this is sort of ridiculous... on the other hand, though, he wasn't that far off. We're more of a Jawbreaker Earth, with a bunch of layers, but the molten iron core does actually shift and fluctuate. This is what causes the magnetic drift that was being observed. That explanation, though, is nowhere near as much fun as the idea of there being an entirely different world inside our own!

Making This Concept Your Own


A lot of Hollow Earth games have a very pulp adventure/steampunk vibe to them, because that's when the concept really hit pop culture awareness. Books, short stories, and movies all trying to capture that pulp era really leaned into the Hollow Earth idea. Sadly, it turns out John Quincy Adams didn't believe it, but an alternate history game where he did, and funded an expedition your characters are now part of is just one inspiration for a Hollow Earth game.

However, if you want to build a really unique Hollow Earth game, I would recommend doing something a little different, and mashing it up with games or genres that we don't usually associate with this kind of setup.

A world without sun, you say...?

For example, is this Hollow Earth a place where all the creatures of horror have their origins? A place filled with vampires, werewolves, the walking dead, and other terrifying beings? Is it instead a place filled with alien races who have lived here far longer than humanity, the center of the world filled with horrors like elder things, gugs, and other Lovecraftian monsters? Did the Earth open up to swallow people at the end of the world in Mork Borg, and now this place is being explored by those who were raised on the poisoned prophecies of the basilisk, finding out there were long-lost truths beneath the surface?

Is the Hollow Earth where this fantasy game has been happening this whole time, and it is through finding the boundaries of this world that our adventurers climb out to the outer shell? And if so, what do they find when they reach the upper crust of a world they thought they knew? Is it the modern world as we know it? A ruined hellscape of frozen wastes representing the edges of Snowhaven? Or something else entirely?

If you've been looking to do something different for your next game, but the previous installment of Using Flat Earth Theories To Fuel Your Fantasy Worldbuilding didn't really scratch that itch, then consider what a Hollow Earth game might look like at your table! Even (or perhaps especially) if the game won't be taking place on Earth at all.

Lastly, before you head out today, consider the following articles of mine as some additional reading/inspiration for this week's post:


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