I love abandoned buildings, underground places (literally underground, not just off the mainstream) and oddball places like aircraft boneyards. Who else likes this sort of thing, and what stories/places/recommendations can you tell me about? What are good cities/countries for this sort of thing?
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Puerto Rico (pic illustrates some of the skills needed to go under as you may find anything in there) is a paradise of underground areas some built by nature others by technology.
The Vieques NASD Roosevelt Roads Base is a great place to explore old bunkers and ammunitions storage facilities located partially or totally underground. It is a fantastic place to record sounds and echoes. It can also be very challenging in many ways so I recommend it only to advanced travelers. Also present on the Island of Vieques are old spanish forts with sealed catacombs and some marine caves on the south side beaches. Puerto Rico itself has so many amazing underground systems that is beyond this answer but you will find a well developed tourism sector supporting explorations there. I am currently in Costa Rica following the trail to a mystical ancient underground temple complex called El Templo de La Luna.
There was a really cool underground salt mine outside of Salzburg that I visited during a business trip a few years back - complete with a train ride, wooden slides and a boat ride - all underground!
An iconic place to see a lot of abandoned buildings is Detroit especially the old, once elegant train station.
The underground Salt Museum in Krakow is wondrous with everything made out of salt: floor, ceiling, walls and all sculptures.
Mammoth Cave in Kentucky is huge as the name suggests and wanders for miles. Many other caves in the region.
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Have a look at two categories we made on Globe Spots, they might be of interest to you!
There's also this blog I quite like, but I think it's no longer maintained: http://www.desolationtravel.com/
I think you should visit Brazil, here we have a lot of places like that. I do not know them cause it's not my type of trip, but I really think that you should search some places here!
There is an underground tour in Sacramento and Seattle. Both sound to be very interesting, but havent been on one of them yet. Thats just what I know.
But I love undergrounds too, especially as Geocacher, you get sometimes into some amazing and crazy spots undeground. But most of them are not really scenic or interesting for tourists, its just more about the way to get to the cache.
in Ingolstadt there was a place, where you could enter the old fortress walls and walk in them. Most of them are now underground and you can walk long ways around the city, but the entrance is now gone, after they built new buildings there...