I like an old railway tunnel we had a few local to us that had some amazing features, crazy the work that went into their construction, unfortunately one of the best is now an aerodynamic testing facility!
There is an old ROC post at the end of our drive, not in good condition and very few original features left. But I have been to some with permission that are still in their original condition!
My other half and I went and looked at the Atlantic Wall in Europe a few years ago, very spooky in some of the complexes there, and the thought of our brave troops being shot at from them as we looked out from the slits was mindboggling!
Love a good urbex, years ago before it became a thing and loads of people started being foolish there were some very good open sites, and I even had permission to go into a few military sites. Now though there are too many Muppets on YouTube thing to make a name for themselves and ruining it for the sensible few. Most sites now have at least perimeter protection and Cctv or security. Also a lot of them have have since been redeveloped as housing or commercial projects!