The Spell of Preservation is the Power of Urt
by HåvardI always liked using both the idea of Mystara as a Megalith, as described in the Gold Box Immortal Set and also supported in the description for Terra in the WotI Immortal Rules and I never saw any reason why the existence of the Hollow World would be a problem for using this idea, especially since the Gold box states that the Megalith called Urt is a unique and somewhat different Megalith than the standard monster type and that the Immortals have made special arrangements with it.
The Power of Urt
The main idea here is that the Spell of Preservation is part of the arrangement that the Immortals made with Urt. Or perhaps a separate arrangement made later on. Of course, Urt is asleep, but the Immortals may be able to contact it through dreams as may others like the Burrowers or even mortals like Druids.
The Nature of the Spell of Preservation
The Spell preserves life forms and cultures that are associated with the past. While Megaliths are connected to the Sphere of Matter, I think the Spheres of Time and perhaps also the Sphere of Thought could be connected to this enchantment. It makes sense that Megaliths are connected to Matter, but it is also interesting to think about how a planets rotations around the sun and around themselves can be seen as an expression of Time. I also find it interesting that the same power that makes the Milenians want to hold onto their cultural values and tradition is also the power that keeps Dinosaurs and Megafauna alive.
Spillage to the Outer World
Another theory of mine is that Lost World areas like the Isle of Dread and the Lost Valley region of Davania etc are the results of the Spell of Preservation leaking into the Outer World. Why is this happening? A result of Burrowers?
Druids
Where does the power of Druids come from? Druidic magic also seems to be linked to Matter and Time primarily. It might also be interesting to have Druids have a special connection to the Megalith.
Urt is dead?
I think the main argument behind this fan idea is that the Gold Box states that the magma and core of a planet are its brain and internal organs. I prefer to latch onto the idea that Urt is different as mentioned in the Gold Box, but if you want to keep Urt dead, many of these ideas might still work. Personally I don't like this idea because it is a bit too sad. Mystara was always an optimistic setting in my opinion.
Mortals Manipulating the Spell of Preservation
Could it be possible for extremely powerful mortals to manipulate the Spell of Preservation? One of the ideas for my own campaign is that a group of mortals are trying to bring Alphatia back into the Outer World. One idea they are exploring is to make use of an Artifact that will make the Hollow World reject the Flying Continent of Alphatia and transport it back to the Outer World to get rid of it. Could this succeed? That remains to be seen. Of course, toying with Immortal level Artifacts has its own risks.