The Island Without Green

Kashima, a tiny Japanese island,was once the most densely populated place on the planet. It was developed as a coal mine in the early 1900s. During WWII, Korean and Chinese prisoners of war were used as forced labourers and over 1000 of them died due to wretched conditions.
After the war ended Japanese workers were used. Schools, restaurants, brothels, and gaming houses were erected. The island with a population of 6,000 was so densely built up with concrete structures it became known as "Midori nashi Shima," or the island without green.

The mine closed in 1974 and the island is now a Unesco World Heritage Site.

More: Atlas Obscura