Abandoned poison gas factory power plant, Okunoshima, Japan
The other is for it's very sordid history. You see, Okunoshima was the site of poison gas manufacturing during World War II. Because of it's size and discreet location, it was the optimum location of creating weapons secretly. The poison gas made here - including tear gas and mustard gas - was used all throughout Japan's occupation of East and Southeast Asia. After the war, the island was quickly abandoned, and is now a tourist destination with a small resort, golf course, a tiny beach, and of course, tons of rabbits.
You don't have to go far to find ruins. Once you get off the dock, and take a right, you soon come across a rather spooky tunnel, and on the other side you see this:
Once you go threw it, you are standing right in front of one of the main power stations that supplied electricity to the weapons plants.
The signs say of course "no trespassing," but that never stops me (or anyone else, as the abundant amount of graffiti inside proves)...
If you are interested in seeing more views of this building (including detailed views of the graffiti), other buildings on the island, and of course the bunnies, I have a whole set dedicated to Okunoshima on Flickr.