>>530043
Don't have pictures, but been around some old forts in the NL and some abandoned shopping mall kind of building and an old Luftschutzbunker (air raid shelter) in GER. I'd advise anyone wanting to do such stuff to read some literature on it first, especially regarding the hazards, such as mold, asbestos, gases, lack of oxygen and collapsing structures.
To avoid most of these, you should get some form of drone with a camera, so you can go where your body shouldn't or simply can't. Of course the range is limited as well, but in the worst case scenario you loose your drone and a bit of money, instead of being trapped somewhere and endangering your life.
As for the places I've been:
In Maastricht there is an old army fort, apparently called Fort St Pieter, where some local dude sends his sheep to graze. It seems there's also some tourism going on with underground tours etc. You can walk around on some kind of dam around the fort and enjoy the nature in the middle of the city, kinda nice. Pic related.
https://www.maastrichtunderground.nl/de
In West Germany there was an old abandoned shopping building that no one used for decades, since the location sucked due to too few people around and the people living there complained about trucks delivering shit, etc. So practically that building was sitting there, rotting away. Been to it a few times, back door was chained but the glass smashed, whole thing was falling apart with trees growing on the roof already. There was someone even living there, found a mattress and some bottles in one of the upper stories.
In North Germany there are some air raid shelters that have been re-purposed as hobby rooms for rent. Buddy owns one and there's a small metal door in the wall, which leads to a chamber filled with some grey sand or dust. We crawled in there for a few meters, but the air was really really weird and so we gtfo there. Oxygen was probably near non-existent and who knows what that grey shit was.
Besides that there were some ruins in the forest, old water collection basins, old bunker and pipe buildings that were sealed off. Nothing of too much interest. In fact, most of Germany has whole bunker systems underground, many of them still in use by military and secret personnel. But that was even the case in medieval times, with cities having whole tunnel systems under them that lead who knows where. They've been restored for modern use with metal doors and locks and everything. Rarely you get to see those when some maintenance on historic building is done.
But even before that, in antiquity, there were underground structures. I know of some underground stone houses in Europe, as well as whole tunnel cities in the Middle East near Turkey and Iraq/Iran. People used to populate advanced underground structures that held thousands of people, with air vents, massive rolling and turning stone doors, running clean and waste water, places for cattle, storage, etc. simply amazing. Pretty much a lost chapter of human history if you ask me.
https://en.wikipedia.org/wiki/Derinkuyu_underground_city
https://en.wikipedia.org/wiki/Kaymakl%C4%B1_Underground_City
https://en.wikipedia.org/wiki/%C3%96zkonak_Underground_City
https://en.wikipedia.org/wiki/Nushabad#The_underground_city