Maunsell Forts, UK
There were built on the Thames during WWII to help defend the UK.
Battleship Island, Japan
Battleship Island in Japan was once a coal mining operation with an entire town built to support it. Then, they closed the operation down, moved the people off the island and it remained for 40 years.
South Georgia Island, Antarctica
This Norwegian whaling station on this island was established in 1904 and closed in 1966.
Takakanonuma Greenland, Japan
Built in the 1970s and closed in the mid 80s. Interestingly, the place is often shrouded in fog for some reason. Very "Silent Hill."
Mirny Diamond Mine, Russia
Lots of mines go under, but some are more impressive in both the scar they leave in the earth and the towns that support their growth. Stalin directed the building of this second largest man-made hole in the earth. Eventually, it just became too massive to dig anymore, and so the town closed down and moved away.
Predjuma Castle, Slovenia
This castle in Slovenia is built into a cave. It dates back to at least 1274. In 1511, it was rebuilt after an earthquake. With over 700 years of violent history, it is said to be very haunted.
The most inhospitable and remote places on earth make for ideal abandoned sites. Left to the elements and loneliness, they become giant beacons of better times and lost industry. The thought of going to both a lonely place and a lonely building is doubly delightful.