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Policing in a technological thaumaturgical alternate universe.
Fantasy novels borrow heavily from aboriginal tales and myths of the people who lived before civilisation was inflicted on them. With organized "culture" came the demonization and twisting of those stories. What remains are vague ideas and wisps of truth that remain in popular conciousness.
One of those ideas is the concept of leylines.
Just like a person has blood vessels to channel the life-sustaining fluid, the idea of leylines derives from the idea that energy also has channels that allows its flow throughout the body to keep it alive. Apply this concept to the Earth, with magic as energy, and you have leylines.
Of course, you know this to be complete balderdash.
Except it isn't.
It never was.
Even during its low ebb of thousands of years, leylines still spawned enough magical beings, powers, and thoughts to titillate the common person. Stories of goblins, elves, fae, evil grannies… all true to some extent.
Scientific thought has no idea of their existence because they did their able best to hide.
Some fell through the cracks, to be sure. But most of them were explained away as hoaxes or covered up.
Until sometime in the late 20th century.
At some point, the magical leylines began to ebb higher, producing more power and stranger effects. Wars, machines, scientific advancement: these were but some of the results of a higher level of magic.
The less known part was the portals.
Portals of effervescent colour and duration, lasting anywhere from a single instant to weeks on end. Some were one-way, others allowed people to go to and fro. Obviously, magical beings did their best to keep these secret, too.
Except that they failed.
By 1993, the majority of people on Earth had already heard about and seen evidence of portals and there was nothing anyone could do to cover it up. It simply gained momentum until no one could deny it without being in denial (fundies).
Of course, 1993 was the year when the Great Lakes area, Mesopotamia, the Outback, the Western Sahara, Siberia, and Argentina exploded.
The amount of energy released completely destroyed everything in 500 to 1500 mi radii at each respective area.
This discharge of energy was seen by some as the Rapture. They were soon ridiculed when the world had the audacity to not end. Instead, this emission of energy proved to have very fecund results.
Living things that grew in these areas would be fortified, more vigourous, more… alive. Plants would be larger and more bountiful. Animals would grow larger and more clever. Sick people would feel better within 1 week of staying and eating in these areas. They were termed "miracle lands" and people being people, made plans to move to these areas.
Those plans fell apart when it was realised that while it was easier to live there, it was harder to make a living when everything constantly try to revert to previous modes of existence.
Wooden buildings would soon grow leaves and branches. Stone would somehow become more massive until they would not fit in their places. Concrete would crumble into lime and sand and water. Steel would rust and crumble away, even when made with corrosion-resistant materials.
Most would-be settlers, disturbed by this, would move back to their cities and normal fields and woods. Those that stayed found out something else the explosions had done.
The explosions had made mass discharges of magical being from other universes. The number of portals grew massively with the explosions. Many soon faded away, but the damage was done. Earth had accepted a large number of magical immigrants.
The magical beings showed adeptness at living in the highly magical environment they found themselves in. All items they created would remain created, instead of violating the second law of thermodynamics.