Google Sightseeing notes an interesting island off western Kyushu, Japan.
Hashima is small (6 hectare / 15 acres), and was owned by a coal mining company, which housed the miners on the island. Writes Google Sightseeing:
When space for the workers began to run out, they built Japan’s first large scale reinforced concrete apartment block on the island in 1916. More concrete tower blocks followed, and by 1959 the population of Hashima reached its peak of 5,259 — an astonishing 1,391 people per 10,000 square metres within the residential district — which is said to be the highest population density ever recorded in the world.
That would indeed be the most densely populated island known: 227,000 people per square mile, or 88,000 per sq km, surpassing the current record holder, Ap Lei Chau, which houses 160,000 per sq mi / 60,000 per sq km. This is all the more impressive given that Hashima was not connected to mainland Japan by bridge.
The island is now evidently deserted.