Speculative fiction really shines when it makes the impossible possible. Not the variety of implausibility that was accidental: that can be overlooked for a good story but is never a plus (oh- there’s no such thing as a 1000 Megaton bomb in the Russian arsenal? Oops). What really grabs me is when an author builds a story around something that seems outlandish and makes it work. Larry Niven is good with that, especially when writing with Jerry Pournelle to give him some engineering ballast: what would it be like to survive a comet strike or how would a modern person take to a trip through Dante’s Inferno?
In posts like Map-Making Pitfalls, I tend to call out what shouldn’t be. But shouldn’t be isn’t the same as can’t be. As I mentioned in that post, casually putting a fork in a river that results in egress to two different seas is lazy map-making. It makes you look careless and who wants a prospective buyer thinking that when they are flipping through your book? But a powerful technique is to take these ‘impossibilities’ and turn them around: okay, this does exit. How do I make it work?
In this exercise lies the seeds of gripping, unique stories that aren’t a knock-off or variant of a hundred fantasy epics that have come before. Take Lord of the Rings. Tolkien started from mythical underpinnings, which gave him the idea of a cursed ring. Next, one can imagine him wondering, how do I make this ring so important, so cursed that it can sustain a long story arc? Because normally, a little bit of gold fashioned into a circle isn’t going cause that much sturm und drang. What if the ring is tied to an evil overlord, one so powerful he could cast the world into endless darkness? But how can the heroes fight such a foe? Maybe the ring contains part of his power, making him weaker than he would be, and so on…
Let’s take the implausibilities of Map-Making Pitfalls as an example. That post already covers ways to dial with a down-stream facing river fork: put something there to maintain the fork and make the distribution of water at the fork relevant. E.g., without the fork, an entire valley will dry up and die. That gives you some seeds for conflict right there.
Looks look at others:
Not enough forests: what could suppress forests in a climate where they would naturally flourish? Disease, blight? Sure, although that might be somewhat impersonal or smack so much of ecological activism as to offend some readers. The sauropods were such voracious herbivores that they probably shaped their landscape. What about some sort creature whose very presence destroys (or at least culls) woodlands? You could have dinosaur-like grazers but that could be a little dull (unless you made use of them as war-beasts like Ollyphants). Since this is fantasy worldbuilding, let’s go with something more exotic: the dominant intelligent lifeform are giants, really, really big giants. They knock down trees much as humans might mat grass. Where they go, humans and trees do not. Now you have the beginning of conflict: human-giant, and a reason why forests are patchy. Similarly, you could have some fire-breathing critters that tend to cull the woodland: maybe dragons prefer open grasslands with grazing herds they can swoop down and eat at will (I think I might prefer that if I were a dragon). Dragons would need vast feeding grounds and could easily keep large areas mostly clear of woods. What other reasons can you come up with for fewer forests?
Gap between forest and mountains: this one is a little tougher. What would keep trees from growing all the way into the mountains? You could go with variations of the “not enough forest” ideas above but have the creatures that keep the land free of trees prefer mountains, so they just clear the mountains and the adjacent areas, something like crazed mountain goats that graze a little beyond the mountains proper. Going a different direction, what if there is something about the mountains that does not lend themselves well to trees? Maybe they are highly volcanic? In Earthly volcanic mountain chains, like the Cascades, volcanoes aren’t erupting enough to do anything more than clear local blast zones, as with Mount St. Helens, or lava fields, although Hawaii does have some relatively barren stretches. But maybe your volcanoes are more active. Makes a good place for fire giants and other such creatures. Maybe the fire giants actually perpetuate this, or even make volcanoes more common by their mining, magic, or some other methods. Now mountains aren’t just a physical obstruction that makes travel difficult: they are home to an alien and dangerous race who, at least near their volcanoes, can harness tremendous power. There would be no wonder now why forests don’t run into the mountains: they run into lava (and harsh lava lands) first.
Compass Point Rivers: I mentioned in my last post that one reason for rivers running along a grid might be that the world overlies a computer or other structure arranged on a grid. For a more fantastic setting, what if your world has ley lines. Maybe the magic on these lines is so strong that it weakens the underlying rock, providing a preferential path for erosion leading to rivers. The same effect happens on earth when erosion is faster along a fault line leading to rivers delineating fault lines (see the San Lorenzo River in the Santa Cruz mountains of California for an example). Such a world might look as if compass roses and nav-lines had been inscribed on it with rivers, seas and bays. One glance at the map and a perspective reader would know something was unique about your world.
Moving beyond map-making pitfalls, how about flying castles? There’s no practical way on Earth that I know of to make a castle float in the sky, Castle in the Sky‘s Laputa not withstanding. But who doesn’t love the imagery of massive piles of stone floating high above? I do and have made use of it in multiple D&D campaigns as well as my current project. So, let’ s turn it around from castles can’t float in the sky to how do I have floating castles in my world? The first option is to have them and not explain them. This is, in fact, a perfectly sensible response assuming you are writing in the fantasy genre where floating castles are an accepted trope. But as always with tropes, scratching below the surface will improve the richness of your setting and will likely generate some great character and plot ideas to use in your story.
Maybe your world, like Avatar’s Pandora, has some rock strata that floats. This would mean that flying castles could only be made from certain places where this strata exists. It would also mean that the continued buoyancy of the castle depends on the strata remaining intact. Perhaps this would not be a real issue in your world, perhaps the bottom of the castle-rock tends to break away over timing, causing castles to age and fall. Or maybe there’s some way to attack this rock layer to neutralize the buoyancy or mine it away (earth elementals?).
Alternately, maybe there’s magic that imparts buoyancy. Life on a flying castle would be very different if the buoyancy was just on the lower part of the block of rock it was built on or whether it was more of a bubble that encapsulated the rock and some or all of the castle. In the latter case, those on the flying castle might be living in low or zero G. Rather than the magic just ensorcelling the bedrock, maybe it depends on enchanting objects or structures embedded into the castle or its supporting rock. Such structures could provide points where enemies might attack as well as an interesting flourish to describe in your story (maybe the structures are gigantic stone rings?)
Or perhaps it’s a matter of lines of magic. I don’t go into this much in my current project, Shadows of the Archons, because it isn’t germane to the story but my thought is that there are ley lines in the world. These used to pass within the bedrock of the world but one of the Archons found a way to pop the ley out of ground. The lines are still anchored in the mountains (where they re-enter) but between the mountains they arc free of the world. As a first step, this allowed the Archon to tap the ley better for use in various arcane arts but he later figured out that he could pull land from the ground and float it into the sky by fixing the slab of rock on the ley lines before popping it up. (I imagine something like a sheet of metal that was flexed downward but popped to flex upward by a sudden force.) What does this mean for the world? In this case, the isles don’t move, they are fixed on the ley (I supposed you could also have them slide along the lines but I like them stuck there like a bead on a knotted string for my story.) My dragons are also able to fly because their wings can catch the ley– which means they can only fly where the ley is accessible.
There are other ways to float rocks in the sky. While they aren’t likely to work with Earth physics, they can provide enough of a rationale to improve reader-suspension-of-disbelief and, more importantly, these details can give you many hooks to hang your story and plot twists on.
In summary, it is best to avoid the unintended ‘impossible’ because that can make you look sloppy, at best, or clue-less, at worst. But figuring out how to make something that seems impossible plausible can be at the heart of memorable speculative fiction.
Related articles
- Castle In The Sky (scrawnygirl.com)
M. Q. Allen 
On the forest/mountain gap, perhaps there could be something in the soil itself that prevents trees from sprouting and growing. Either a naturally occurring chemical, or an evil spell to keep the approaches clear.
I do know that in areas with volcanic hot springs, the sub-surface heat can kill tree roots. This is usually only within small areas close to the springs, however, and it allows grass and smaller vegetation to persist.
In a darker fantasy, perhaps there are demons or other fell creatures that suck all the life out of the land, so that nothing can grow at all. Even animals would die. Although, you could do something fun with an ecosystem where only tiny creatures (insects and spiders) could survive.
Great ideas! The evil spell would give heroes something to try to undo or defeat. On the hot springs, maybe on earth it tends to be small areas but perhaps in a fantasy world, there are hot passages created by some sort of creature. These passages might be close enough to kill vegetation. A sign of attack might by a progressing line of dead plants.
I also like the fell creatures sucking the life from the land. I remember something like that from the Amber Chronicles but for the right story, it can certainly raise the stakes (defeat the bad guys or life as the heroes see it will become impossible).