The pattern I am talking about is a slow build toward a Hollow Earth setting. The assumed setting of the Early Modern Era was rife with legends of a world inside our own. A quick internet search will show that there are people who still believe there is another world inside ours and a secret history to go along with it. It's a weird location with almost limitless potential. Because any characters exploring the Hollow Earth would be from the historical world of the Early Modern Era there is still a "normal" experience for players to balance against the weird. The proximity to the real world also allows for some opportunities for extremely ordinary things from our world to be found in the hollow earth. All these things allow for a completely new world full of strange new things to explore while still keeping to the basic tenets of Weird Fantasy Roleplay.
The most famous Hollow Earth fiction is probably Edgar Rice Burroughs' Pellucidar series. One of things that captured my imagination about Pellucidar from a gaming perspective is how the constant daylight affects the perception of time. Here in Canada it is possible to live far enough north to experience "the midnight sun" during the summer. I don't live that far north (thankfully!) but I have friends who have and other friends who currently work up there a few weeks at a time. It can mess with a person's circadian rhythms. It has even been known to drive people mad. It's not a far jump from there to Burroughs' ideas of time having no meaning in Pellucidar.
Burroughs took it a step father though. In his stories the perception of time is a necessary part of its working. Because the characters could not perceive time passing they could not be affected by it in the same way as a person living on the outer surface would be. This means PCs taking a "night's rest" need to set a watch or they might sleep for the equivalent of a hundred years. It also means they consume resources such as food and water at irregular rates. Going inside a space where time is important in terms of how long a torch lasts might be shocking after they get used to unfettered time in a wilderness crawl. The short and long rest mechanic from 5e D&D could fit well into the fluid time of Pellucidar. Let's hope WotC gets on with their update to the OGL so it can be ported in.
The mutability of time makes things a bit difficult for a GM who is tracking the actions of other factions operating concurrently to the PCs. Things like distance and resources will have to determine what can happen in the background while the PCs act but with time the way it is pretty much anything can happen. The beauty of LotFP is there is already a blueprint for dealing with these kinds of issues in The Monolith From Beyond Space and Time. Time in that adventure moves at different rates for different characters. Distances are relative and it takes quite a lot for the players to wrap their heads around what is happening. So much of the groundwork for Pellucidar's strange time is done for LotFP.
Another thing I love about Pellucidar is that the inner earth is protected from the trauma experienced by the outer world. That means there are still dinosaurs! Besides the fantastic that could exist hidden from our view in the hollow earth there are all the creatures from every era of earth's history down there. All the creatures of legend that seem to appear and disappear like Sasquatch/yeti, the slender man and the moth-man could all be residents of the hollow earth that somehow made their way to the surface as well. Again, the possibilites are endless. The hollow earth with its disconnected time and entrances through the ground could explain the legends of faerie mounds where people entered the land of the fey only to return to a time hundreds of years after the one they left.
When it comes to dinosaurs though, LotFP has it covered! The infamous adventure setting Carcosa has dinosaurs but it's the still-under-development World of the Lost that I think is being written by Rafael Chandler that leads me to believe a hollow earth full of dinosaur shenanigans is on the way. It's clearly inspired by Conan Doyle's Lost World and is a mere step away from Pellucidar.
Getting back to Carcosa, the depravity of the snakemen of that product matches up well with some of the terrible creatures of Pellucidar. The low placement of humanity on the Carcosan food chain also fits in well with the character of most hollow earth stories and is just as ripe to be upset by enterprising PCs as it was in the original stories set in Pellucidar. The Judges Guild style hexcrawl format used in Carcosa also would serve well to cover a section of the hollow earth in detail.
A Red and Pleasant Land by Zak S. deserves mention in this progression toward a hollow earth setting as well. Its Land of Unreason is a world apart with its own rules and is difficult to access. It's not much of a reach for players to adventure in the hollow earth after their characters survive (or don't survive) A Red and Pleasant Land.
Besides all this conjecture and guessing on my part there is the Thulian Echoes adventure! In that one by Zzarchov Kowolski there is an explicit stairway to the hollow world. The adventure gives no clues as to what the hollow earth itself is like but the journey is described as a long one and there are random encounters that include the corpse of a Triceratops! In Thulian Echoes there are creatures down in the earth's crust inspired from other works such as the Vril from Edward Bulwer-Lytton's Vril, the Power of the Coming Race (originally published as The Coming Race in 1871). There are also devolved versions of the elder creatures from H. P. Lovecraft's At the Mountains of Madness.
I like the inclusion of the creatures from At the Mountains of Madness. It could mean that the cyclopean city set in Antarctica was an outpost of a grand subterranean civilization that still exists in the hollow earth. With time as mutable as it is, little time could have passed for that civilization and they might be still wondering what happened to their city they only recently lost contact with.
Image by Kelvin Green
The disconnect with the progression of time between the out and inner surfaces of the world means characters could encounter lost legionnaires from Rome, people of Egypt or any era of history. A GM could even mess with players by having their characters encounter explorers from an era a hundred or more years after their own to show how they've lost time on the inner surface.
The time thing gives me an idea for an interesting location. A mad old wizard's tower where the incredibly ancient man wears the ravages of time on his wrinkled skin as he tends to water clocks and endlessly winds the clockwork of the tower itself in an effort to bring time to the hollow earth. It could create strange time effects near the tower and give rise to a host of magic spells based on controlling time in the tower's library.
I like the idea of a hollow earth or Land of the Inner Sun campaign setting. In fact, if James Raggi of Lamentations of the Flame Princess doesn't get the ball rolling on this thing in some official way I might have to go ahead and do this one myself.