A new discovery was made that altered the course of human history. Great scientists worked out a way to travel to another world. An actual... other world. The potential of this was almost limitless. It would be an event that changed the way some towns- and even countries worked.
A certain curious energy was found that seem to breakdown the barriers of two dimensions opening a small window or gate. This created a dimensional quantum event that allowed a particle to exist in two spaces at the same time. There were two modes of operation at use:
The first method was to create a simple tunnel or doorway, directing the field like a magnifying glass to a central point with the size being the only limit. Optimal portal sizes tended to be about twenty feet in diameter and required a great deal of energy to 'activate' (with far less energy needed to keep open once it was open). This was usually not an issue for the Earth-side which could supplant the power needs of the initial burst with nuclear energy or other high technology means... but the door required a similar setup on the other side, powered by a different energy field. By coincidence the same experiment was being done in the other universe forming a bridge...
… and thus contact was made.
People from a parallel Earth! To live and trade with, even exchange ideas as well as explore peaceful scientific (or magical) endeavours.. Though they were not human... in fact almost every other fantasy race one could imagine was there. They were as curious of humans as much as the humans were. It seemed that Earth could make friends with an entire world right in their backyard!
The potential of this technology was explored further, pushing the limits on both worlds.
It was found that the gateways were not the only consequence of manipulating this dimensional energy. It was also possible to swap permanently, entire areas of Earth with the alien world. A circle of towers were placed in careful alignment with a similar setup in the other world causing the two areas to swap places. An entire town from that world would end up on Earth, while the Earth town would end up on the other side. The upside of this was that human technology tended to work as expected on the other world... and the strange energy would work field on Earth would allow for some curious sway on the established laws of physics. This caused some... interesting political debate on if such swaps would be useful though it has been quite rare to set this up. Only a few successful swaps were done and only temporary. Governments were keen to try and have areas to experiment in neutral places creating such circles away from towns, but these tended to lack the vital energy. For some reason towns sprang in the other world that had a higher field of this energy like a towns to a stream on Earth... and both tended to run in parallel. It was rare for a town to pop up in the other world in absence of a town on Earth... making it almost impossible to swap a vital spot without sacrificing a town in the process to make it worthwhile.
With these considerations in mind, very few governments were comfortable to swap entire towns but sometimes a temporary swap was done to facilitate further research with the consent of the people (or at least majority in democratic states.)
The races of the other world were numerous taking on all shapes and sizes. They also varied by geography and... interestingly enough, tended towards the mythological expectations of the society it was near to. For instance, there were centaurs around the greek islands, and Fox-like Kitsunes in Japan though they tended to move around as much as humans did on Earth.
Of course humans didn't believe in magic but whatever this energy was they were getting better at manipulating it.
There were protests on Earth among the concerned... that humans were now starting to mess with dimensional energy and a tragedy could unfold with this new and potentially dangerous technology. It could destroy the Earth or cause environmental damage across both worlds. Such concerns were not without merit.
On the other hand... suddenly there was an entire world that one could trade with minimal effort bringing many precious resources and ideas with it. Since human technology didn't work there it was... mostly safe from potential environmental damage. That both sides needed to agree to a portal being opened was also a reasonable insurance.
The population of the other world was much smaller then Earth, estimated to be about only a billion compared to the 9 billion on Earth (and given that was among tens of thousands of different species... its possible that each species had an even smaller population). With the economics in mind, other countries began exploring this technology becoming more powerful.. it naturally lead to entire nations quickly establishing ties and relations with this other world as best they could for their own corner of the new world.
Soon enough there were now families being born between the two worlds... though this seemed to bring its own... unusual issues with regards to the energy, especially if the families involved non-human lines.
Geography was interesting in that there were a few reasonably consistent rules. For one thing the lay of the land of this other world pretty much matched almost identically that of the Earth. There was usually something physical in all physical points, though there were interesting exceptions to the rule. That meant buildings tended to be the same general build, a television might have a portrait or mirror instead given the other world lacked such technology. Places that were completely natural looked the same as did the volcanos and oceans.
Though the sahara desert was full of ocean water in their world and there was at least one extra continent that humans couldn't reach easily and the locals were reluctant provide information on it.
In the end some permanent areas of trade were created that occupied both worlds at the same time with gateways in between. A person could return to the different reality based on which road they used. (There was a pretty nifty train system that was owned by both places, the train model actually changing as it went through the system.)
Though there was one element of this dynamic that was very important; both sides had to agree to having their gates open. If one side revoked it then no side could go to the other. As a result politics remained as friendly as possible (often outright staying away from the affairs of the other world just in case.) Earth felt it was imperative to have a door open to another world almost at any cost...
