This is probably as close to an urban explorer’s dream location as it gets: Battleship Island, Gunkanjima in Japanese, an abandoned island off the coast of Nagasaki. Today, it can be easily reached by guided tours from Nagasaki port.
Access to it is highly restricted, however. Visitors are confined to several hundred meters of fenced walkways that had been built when Gunkanjima was opened for tourism in 2009. Since the island has not been maintained for several decades, some structures collapsed over time. To prevent further damage, a few areas were already reinforced with concrete. Since a full reopening would require substantial investments in restoration and safety, it is rather unlikely.
The island’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 Gunkanjima’s former residents like tube TVs or discarded soda bottles.