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.

Overall, the Japanese case is weak, though the government argues otherwise. According the the UPI article,

Last year Shintaro Ishihara, the nationalist Governor of [Tokyo], was photographed kissing its dwindling earth. The problem is Article 121 of Part VIII of the UN Convention: “Rocks which cannot sustain human habitation or economic life of their own shall have no exclusive economic zone or continental shelf.” Even Mr Ishihara would balk at living on Okino-Torishima, although there is talk of setting up an electricity plant to establish “economic life”.

(Perhaps the Japanese have heard this Malay saying: “Where good seed falls into the sea, one day an island may appear.”)

On a blog, an angry poster listing Japan’s historical offenses against Korea leveled this curious accusation: “The Japs has made the land to be officially named ‘Half Island’ instead of ‘Peninsula’. Kingdom of Forgery!”

“Hanto” is Japanese for “peninsula,” and means “half-island” in English. That is of course the same meaning as “peninsula,” which is Latin for “almost island.”

The oddest part of this complaint is that in Korean the Koreas occupy a “bando.” What does this mean? An American university notes that “the Korean term for peninsula (bando) means literally ‘half-island.’”

(”Half-” or “semi-island” is the term for peninsula in many languages. Perceiving a protuberance of land this way does not seem self-evident, and I wonder what peninsulas are called by peoples for whom islands were the center of things, not isolated fragments, for instance in the Pacific?)

The practice of sending someone to live on disputed small islands is partially an assertion of their island status: an attempt to prove that they are places rather than things, islands not rocks.