There's an island in Japan that once had a population density of 216,264 people per square mile but is now completely abandoned.

The whole island of Hashima is an abandonded ghost town. Until 1974 the island was a coal mining facility bought by Mitsubishi in 1890. It now has huge concrete buildings that are completely abandoned now and a sea wall. They actually built Japan’s first large concrete building with nine stories. 

They also built a block of apartments in 1916 to house all the workers, most of whom were forcibly recruited from Asia. South Korea claims that there were 500 forced Korean workers between 1939 and 1945. 

The 15 acre island reached its population peak in 1959 with 5,259 people. That translates about 216,264 people per square mile for the whole island. In the 1960s, petroleum took the place of coal mining on the island. Coal mines were shutting down all over the country in the 1960s. 

Mitsubishi officially closed the mines in 1974 and the island became barren of people. Now it is justly called Ghost Island. After being closed for 35 years, travel was made possible to Hashima again in 2009. Today the island is becoming increasingly popular, especially with ruins enthusiasts. 

The housing remnants represent the period between the Taisho era and the Showa era. The island hasn’t been maintained, though, and some of the buildings have collapsed and others are about to collapse. Some exterior walls have been repaired recently, though.

(Source)