The haunting silhouette of "Battleship Island" rises up from the sea, an abandoned testament to what was once the most densely populated city on earth.

Thousands of men, women and children lived and worked on the island, harvesting undersea coal mines that powered Japan's rapid industrial rise from the late 19th century.

But over the years, it made less and less economic sense and in 1974 operator Mitsubishi Mining abandoned the site, just off the coast of Nagasaki.

Known in Japanese as Gunkanjima, it was the villain's lair in the 2012 Bond film Skyfall and won UNESCO heritage status three years later, in 2015.

But not everyone is happy about the attention being given to the sea-wall encircled island, a popular tourist destination whose shape resembles a naval destroyer.

The former city's crumbling concrete walls, smashed windows and rusty iron support bars harbour a dark secret -- Chinese and Korean workers were once forced to work here, slaves to their colonial master Japan.

"Gunkanjima is an evil place," said Zhang Shan, vice president of the Chinese Forced Labour Association.

"(UNESCO status) was a desecration and a shock for the victims."

It holds different memories for people like Minoru Kinoshita, 63, who was born on the island.

"I've come here often and every time I see that my hometown is falling into an increasing state of decay," he said.

Until the age of 13, the 6.3 hectare (16 acre) island -- packed with a school, swimming pool, open-air market, a hospital, small prison, and rooftop vegetable gardens -- was the only home that Kinoshita had ever known.

Kinoshita, whose father was the local movie theatre projectionist, remembers a magical place for kids, a dense labyrinth of buildings perfect for playing hide and seek.

Inside, four or more people lived together in tiny tatami-mat rooms. Residents boarded up windows when violent typhoons lashed the island.

Battleship Island's population peaked at nearly 5,300 around 1960. It was an offshore version of Europe's once-booming mining towns. And like its overseas counterparts, the work was anything but pleasant.

The mines operated 24 hours a day in rolling eight-hour shifts.

Up to 1,000 metres below sea level, men toiled in cramped and stifling spaces where they had to defecate into small holes that they dug themselves.

"The air was thick with humidity. It was sticky and the coal dust mixed with our sweat so we were black from head to toe," said Tomoji Kobata, a 79-year-old who worked on the island for about a year and a half during the early sixties.

More than 200 workers died in accidents over the years. Others suffered from silicosis, a work-related lung disease.

Some were not there by choice.

Japan brutally occupied the Korean peninsula and parts of China at different times in the first half of the 20th century, and sometimes used their workers as slave labour in the years leading up to and during World War II.

The Chinese forced labour association protested to UNESCO over the heritage status, but got no reply, Zhang said.

Reliable figures on the number of forced labourers are hard to come by.

Mitsubishi Materials, a descendant of the original operator, has said it is going to place a memorial at some of its former mining sites to honour forced labourers.

The company has awarded nine former Chinese labourers forced to work at other locations about 100,000 yuan ($14,500) each. A couple of other settlements are underway.

In 2015, Tokyo said it would take steps to ensure visitors understand that many Koreans and others were brought to the island and forced to work under "very harsh conditions".

Tourist brochures mention their plight, and guides remind visitors that it was not only Japanese who toiled below ground and sometimes died.

Former resident Kinoshita hopes UNESCO status will mean more funds to restore the dilapidated buildings, and keep alive memories of the day in 1966 when his family bid the island a final farewell.

"When we got on the boat to leave, I saw my friends waving a banner with my name on it," he remembered.

"There was also a message: 'Never forget our island!'"

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MITSUBISHI MATERIALS