Few places in the world have a history as odd, or as poignant, as Gunkanjima's. The tiny, fortress-like island (also known as "Hashima") lies just off the coast of Nagasaki. The island is ringed by a seawall, covered in tightly packed buildings, and entirely abandoned - a ghost town that has been completely uninhabited for more than forty years.

In the early 1900s, Gunkanjima was developed by the Mitsubishi Corporation, which believed - correctly - that the island was sitting on a rich submarine coal deposit. For almost the next hundred years, the mine grew deeper and longer, stretching out under the seabed to harvest the coal that was powering Japan's industrial expansion. By 1941, the island, less than one square kilometer in area, was producing 400,000 tonnes of coal per year. And many of those working slavishly in the undersea mine were forced laborers from Korea.

Even more remarkable than the mine was the city that had grown up around it. To accommodate the miners, ten-story apartment complexes were built up on the tiny rock - a high-rise maze linked together by courtyards, corridors, and stairs. There were schools, restaurants, and gaming houses, all encircled by the protective seawall. The island became known as "Midori nashi Shima," the island without green. Amazingly, by the mid-1950s, it housed almost six thousand people, giving it the highest population density the world has ever known. And then the coal ran out.

Mitsubishi closed the mine, everyone left, and this island city was abandoned, left to revert back to nature. The apartments began to crumble, and for the first time, in the barren courtyards, green things started to grow. Broken glass and old newspapers blew over the streets. The sea-breeze whistled through the windows. Now, fifty years later, the island is exactly as it was just after Mitsubishi left. A ghost town in the middle of the sea.

From 1974 to 2009, the island was officially closed to all visitors, but recently the intriguing site has been re-opened to organized tours. Advocates are currently pushing for the tiny island, with its remarkable history, to be added to the UNESCO World Heritage list.