This is probably as close to an urban explorer’s dream location as it gets: Battleship Island, Gunkanjima in Japanese, a derelict island near Nagasaki.
As a tourist you can reach the island by guided tours from Nagasaki port these days. Access to it, however, is rather restricted. You will be confined to a few hundred meters of fenced walkways built when Gunkanjima was opened for tourism in 2009.
As it has not been maintained for several decades, the island’s structures have started collapsing. Some areas were already reinforced with concrete, in order to prevent further damage. A full reopening is unlikely, though, as it would require a substantial investment in restoration and safety.
Gunkanjima’s warship-like shape is a man-made built-up that was done when it served as an undersea coal mine. In its heyday, it was home to more than 5,000 miners, making it the world’s most densely populated place. In the 70s, its coal reserves were coming near depletion. As a result, the mine was closed and Hashima Island, as it is officially called, vacated by all its residents.
With its street view technology, Google provides extensive coverage of the place. It lets you peek into the buildings, even showing remains of the island’s former residents such as tube TVs or discarded soda bottles.