[Author's note: This story may appear incomplete. It is not.]
The structure was first discovered when the king had sent out a large group to explore for a location to mine for more copper for their bronze; the kingdom wasn't running low on it or anything, but a particularly rich area of the tin mines had been discovered recently, producing more than was necessary with the current supply of copper, and the potential for enrichening the supply of bronze in the kingdom held the king's eye.
A barren, rocky portion of the land was chosen as the first place to search, because the lack of dirt would mean less hassle for mining and meant that mining the land wouldn't interfere with potentially using it for farming. Exploration, of course, found no copper. It was as barren of riches below the surface as it was above.
The design of the structure was as if they were trying to make it boring: everything was uniform, grey, and stony, except for the doors, which were uniform, grey, and metally. But the construction was impecable: each corner a perfect angle, each surface perfectly flat—just a shame such skill was used for something as mind-numbingly boring as this.
When the explorers first cracked their way through into the underground structure, instantly did much speculation spread amongst the group about what could be found lying inside. The speculations ended up dying down rather quickly; upon opening the door to the chamber they had burst into, they found themselves in a grand hallway going seemingly endlessly in both directions, lined with the same doors over and over again.
A group was sent in each direction. Initially, they opened the doors to each chamber, but this halted rather quickly as they realized that each one contained the same thing: a single container taking up most of the room, tightly secured, in which was a plain grey substance of no interest to anyone.
Instead, each group just went in their direction, counting the number of chambers (there wasn't much else to do) and wondering what they would find at the end. One group found another hallway, parallel to the one they came down, but similarly seemingly endlessly, and a staircase going down, again seemingly endlessly.
The other group found a chamber unlike all the others lying right at their end of the hall. When they entered, they saw that what when they were walking down the hall they first thought was the wall on the other side of the room was in fact the back side of a great big sign, with text marvelously crafted upon it, but written in a barbaric mutilation of the explorers's common tongue. They could not understand what was written upon it; they decided to copy it down to figure out later, and then they decided to meet back up with the other group and leave.
After the explorers left, they continued their exploration for a location for copper mining. But illness soon did strike many members of the expedition, and so they had to return back early, still having yet to find a suitible place. It was a disappointing way to return, bringing back so little, but it was what had to be done.
A team of wise people was assembled to attempt to understand the strange piece of writing originating on the sign. When it was fully decoded, it read as follows:
If you continue forward from this point, you will find many rooms of storage.
Nothing valuable is stored in these rooms; the only thing stored here is waste.
But this waste is harmful. This is why we disposed of it: we did not want it to cause harm.
The nature of this waste is complicated. If you do not understand it right now, leave it alone; if you do not interact with it, it cannot bring you any harm.
As your society's knowledge of the world increases, you will eventually know what this waste is. We hope you may surpass our knowledge and find a way to make good use of the waste.
But until you completely and entirely understand what this waste is, seal this place back up.
We hid this place randomly to make it as unlikely as possible you would stumble across it. Now that you have, remember its location, and do not allow anyone to come back in here.
The less you interact with this waste, the safer you will be.