The world as we know it is separated by two sides: Underside and Frontside. The Frontside is where humans reside, the Underside is where DAEMONs reside. Think of it as two sides of the coin. One is Heads, one is Tails.
While normally you can't reach Tails from Heads if you were merely a tiny speck on the coin, if you made a hole, you could easily access the Tails side of the coin.
Yes, in the underground, there are entries to Underside. Yet, Underside is an extremely volatile and dangerous area for ANY magician. The Underside not only is littered with daemons, rather hideous creatures, but the atmosphere is heavy with pure magical energy, to the point of being suffocating to breathe. Anybody who ended up in the Underside never came back; and the caged test subjects that were retrieved from the Underside - they had their bodies distorted and skewed beyond recognition; yet it appears that the test subjects were still conscious and cognitive (test subjects were later put down).
And so, there are regulations on how deep the mine can be dug to assure human safety.
Or, there should be.
Underground, due to being the separating point between Frontside and Underside, contains valuable materials and minerals (as they have magical capabilities that are left as the trace from the magic emanating from the Underside) such regulations are often ignored in worse-off regions that aren't highly regulated.
And because of that technicality, there are two types of mines.
One is considered "the earner" which is a mine that is integral to the local area for the economy and provision.
And the other is "the laborer"
Those are the mines with the highest volatility due to closer proximity to the Underside entryways. The resources gained from the mines are sent to the designated facilities, and the workers have minimal pay as a result. The workers are not merely just locals who are here trying their best - but prisoners and the desperate. Those are considered expendable, as such those mines are NOT regulated, and in fact are more encouraged to dig further to the entryways for the magical research purposes.
And it keeps the human trash away. No one will weep for them anyway.
The entryways to the Underside are described as ethereal and mesmerising. There is a sheer "membrane-like" barrier that is easy to enter, but hard to exit. They usually host research sites, but they can also be barren, especially newly discovered entryways or neglected entryways.
It is said that the barriers are maintained by none other than the Wise King of Certainty who once banished Daemons to the Underside. While the records of such figure are now considered merely a legend, some (noted to be on lower end of mental stability) swear on their heart that they can hear his voice, driving them insane.
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"A new entryway is discovered in Praeya, Mr. Emerade."
"Hmm... That's west of Grinwick, no?"
"Yes, it's a rather isolated village from the rest of the vast country. Some preliminary reports say that this village doesn't know what magic is, yet they seem to be familiar with adherents and daemons, probably due to that entryway now seeping magical energy. They refer to them as monsters though."
"That's interesting... I actually want to visit it myself."
"Are you sure, Mr. Emerade? You won't send in the specialised crew?"
"Indeed. I actually want to break out of the tedium of a researcher. While I have a rather comfortable life, I've always been fascinated by on-field research, so might as well give it a shot."
"Alright then, Mr. Emerade. Be warned though, this is a rather unruly village, as it does facilitate convicts."
"I'll be fine I believe. I have defeated many magicians in my time, so I have good enough capabilities for self defense. Thanks for your concern. Well, I better get prepared for the trip then. As my trusted advisor, I will be relinquishing my leadership to you for the time being. Understood?"
"Understood..."
Mr. Emerade leaves the office, eager to explore a whole new world to himself, and along with it, he decided to bring along his beautiful daughter - Sol. She was a rather sheltered girl, so it's good to expose her to the outside world beyond her home, the car and her school.
"...
My job here is done.
I can't believe how naive he is.
And I thought I would fail.
Well, it can't be helped, huh?"