"You are a group of polar researchers and Danish soldiers currently stationed at Station Nord at the northern tip of Greenland. One day, there is an earthquake with some rather strange seismic readings that originates from somewhere nearby - and there are reports of similar earthquakes from around the world. You should probably check the location of epicenter out."
And what our bold explorers - which is to say, the player characters - discover at the epicenter is one of 36 magical gates that have opened up around the world. These lead to another world - a fantastic world, with magic and monsters and nonhuman beings. Or rather, as they will eventually discover, a fantasy Earth, for while society, culture, and technology are vastly different, the geography will have some strange similarities:
So what is going on here? Well, this world used to be a fairly normal (at least in terms of geography) "parallel Earth" from the GURPS Infinite Worlds multiverse. However, about 12,000 years ago the fabled continent of Atlantis destroyed itself in a magical cataclysm, which (a) turned the world into a magic-rich "High Mana" zone and (b) flipped the poles, which means that the ruins of Atlantis can be found at the new North Pole (where 40°W, 35°N used to be).
This caused the expected mass extinction and severe flooding, as the ice caps of Greenland and Antarctica melted (Australia, now located near the new South Pole and being much smaller than Antarctica, can only hold half the ice that Antarctica could, so the water level is still about 30 meters higher). However, in the long run it also had its beneficial side effects - staple crops like Middle Eastern grains and American corn and potatoes had a much easier time spreading around the entire globe than in our world, and the Americas are much less isolated than they were in our world. The former Polar Sea is now one of the world's premier trade regions, with its connections between Europe, Asia, and West America, and the Bering Strait is among the best prime real estate on the planet. Furthermore, magical mutations from mana storms helped repopulate the ecosystem after the pole shift, although this is little consolation to the settlers facing mutant penguins and seals which dominate the vast interior of newly-fertile Antarctica.
The technology has only reached a "medieval" level (TL3 in GURPS terms). However, the practice of magic is fairly common. Simple spells (those without prerequisites) are in widespread use and can be learned by anyone who bothers to learn them. More advanced spells are mostly taught within magical guilds and other organizations who are very secretive about their knowledge and seriously frown on anyone trying to muscle in on their monopoly. This knowledge is usually taught in the form of a Magical Style - but few will get the opportunity to learn spells from more than one of them.
This was the status quo. Meanwhile, one Albert Garner Kavanagh III - one of the Grand Masters of that notorious transdimensional society of mages and monsters, the Cabal - decides he wants to see what high-tech research and technology can do when they come across magic (traditional Cabal magic research is so hidebound). For this reason Kavanagh moves major parts of his operations to a low-mana "modern-day Earth" and carefully stages secret takeovers of companies and research institutions most likely to start dabbling in magical research when they come across clear evidence of it (corporate takeovers are so much easier when you are the only one with access to Mind Control spells...)
Then he arranges for 36 Gates - one for each Decan of Cabal lore - to open across this world, connecting it to the pole-shifted Earth. This has a few repercussions beyond opening access to a fantastic world where magic is demonstrably real.
The Modern-Day Earth starts out as a "Low Mana" world, which means that only people with the Magical Aptitude advantage (i.e. "mages") can cast spells, and even these will have considerable difficulty in doing so. Most people in this world with a scientific education believed that magic was in fact impossible. After learning that this is not the case they might wonder whether the world was raised to "Low Mana" through the opening of the Gates... or whether casting spells was actually possible before that date and was kept secret by its practitioners.
The Pole Shift Earth starts out as "High Mana", which means everyone can cast basic spells with no undue difficulty, and magical energy regenerates fairly quickly.
However, after the opening of the Gates, the mana starts flowing out of the Pole Shift Earth and into the Modern-Day Earth. This means that there are widening zones of "Normal Mana" around the Gates on both worlds. This is a fairly slow process, and will likely take more than 75 years to complete for the entire world. In fact, the total area that gets added to the Normal Mana zone around each Gate each day is constant, which means that the radius of the zone initially increases rapidly - but then its expansion slows down quickly:
1d: 12.72 km
2d: 17.99 km
5d: 28.45 km
10d: 40.23 km
30d: 69.69 km
60d: 98.55 km
90d: 120.70 km
1y: 243.07 km
2y: 343.75 km
3y: 421.01 km
5y: 543.52 km
10y: 768.65 km
Thus, on Modern-Day Earth the zones around the Gates will see a lot of economic growth by those who wish to make use of magic - while the disruption the rest of the world will be relatively small for the time being, as it will remain difficult to cast spells. Meanwhile, on Pole Shift Earth lots of ordinary people living close to the Gates will loose access to simple, everyday magic - but the inflow of modern technology might compensate for that.
So where do the Gates appear? I actually made this as random as possible, by creating a list of all countries (plus Antactica) by their surface area and turning this list into a random table (and rolling d1000000000 36 times...). Here is what I came up with for each Decan (I will also list the magical associations here, as defined in GURPS Thaumatology, p. 248):
Decan (College/ Path): Location
Agchoniôn (Sound/ –): Russia
Akhouiy (–/ Luck): Lybia
Akton (–/ Spirit* (dealings with demons)): Antarctica
Alath (–/ –): Antarctica
Alleborith (Meta-Spells/ Spirit* (general- purpose wards, etc.)): Ukraine
Anatreth (Movement/ –): Mozambique
Anostêr (–/ –): India
Arôtosael (Making and Breaking/ –): China
Atrax (Food/ –): Antarctica
Axiôphêth (–/ –): India
Barsafael (–/ Spirit* (dealings with enigmatic powers)): Greenland (Denmark)
Belbel (–/ Spirit* (dealings with ghosts)): Saudi Arabia
Bianakith (Body Control/ –): United States
Buldumêch (–/ –): Brazil
Charchnoumis (Animal/ Form): Antarctica
Eneuth (Fire/ Elements (fire effects)): Russia
Harpax (Plant/ –): Russia
Hephesimereth (–/ –): Antarctica
Ieropaêl (Earth/ Elements (earth effects)): China
Isrö (Air/ Elements (air/wind effects)): India
Iudal (Gate/ –): Algeria
Kumeatêl (Enchantment/ Spirit* (dealings with “worldly” entities)): Sudan (North)
Kurtaêl (Necromantic/ Health (harmful effects)): Tanzania
Marderô (–/ –): Egypt
Methiax (Illusion and Creation/ Cunning): Russia
Naôth (Communication and Empathy/ –): Peru
Nefthada (–/ –): Kazakhstan
Ouare (Technological/ Gadgets): Canada
Phoubêl (Light and Darkness/ –): China
Phthenoth (Healing/ Health (healing effects)): Canada
Roêlêd (Protection and Warning/ Protection): Brazil
Ruax (Mind Control/ Dreams): Philippines
Sahu (Weather/ Nature): Greenland (Denmark)
Saphathoraél (Water/ Elements (water effects)): United States
Sphandôr (Knowledge/ Knowledge): Peru
Tepsisem (–/ –): Antarctica
Why determine the locations randomly, as opposed to just where I think it would be cool?
Because this forces me to think "outside the box" instead of just following my own biases. In many forms of fantastic fiction set on Earth, the most interesting things happen in the writer's home country - consider how many Hollywood movies with fantastic or science fiction elements play out in the United States. By making the Gates open up all over the world randomly, I can avoid empowering the culturally dominant nations more than they already are, and this will also force me to research regions of the world I am not familiar with. While the campaign will start in a Europe-dominated region (i.e. Greenland), the Gates belong to the whole world, and hopefully the campaign will reflect that.
And speaking of "culturally dominant", I think I will try to avoid pandering to the stereotypes of Western Tolkienesque fantasy too much. While there will be fantastic races and creatures that might resemble elves, dwarves, and similar such elements if you squint a bit, they should fit elements from other myth complexes just as much if you squint in a different way. The Pole Shift Earth has had 12,000 years to distinguish itself from our world, after all - and its differences from Western cultural norms (fantastic ones or not) should reflect that.
These are some of my initial ideas before I start looking at the individual Gates and their surrounding regions in detail. What are your thoughts?