Offshore from Nagasaki, Japan, lies a concrete island known as Hashima, or Gunkanjima, which translates to "the armored island" because of its shape, resembling a warship. Situated about 18.5 kilometers southwest of Nagasaki, the island was once the most densely populated place on Earth. At its peak in 1959, it had a population density of 83,500 people per square kilometer, with 5,259 residents living in tightly packed conditions. The island featured housing, two schools, a hospital, a gym, a cinema, 25 shops, a temple, a pachinko parlor, bars, and even a brothel. This extreme density was achieved through vertical construction, as the limited land area required multi-level buildings to accommodate the population. Originally a small reef, Hashima's potential was discovered by Mitsubishi due to a coal deposit beneath the ocean floor. The company built concrete walls around the island to reclaim land from the sea and turned it into a major industrial site. In 1916, Mitsubishi constructed what is considered the first reinforced concrete building in Japan — a seven-story structure to house the growing number of workers. This marked the beginning of Hashima’s transformation into a bustling industrial hub. The island’s decline was swift. On January 10, 1974, the coal mine closed, and on April 20, the boat service connecting Hashima to the mainland ceased, leaving the island abandoned. Within three months, all the residents — families, teachers, merchants, and others — evacuated, leaving behind their homes, furniture, and personal belongings. The departure was so sudden that many took only what they could carry, as if expecting the island to reopen soon. Over time, without human care, the structures deteriorated due to exposure to the elements. The concrete walls cracked, water seeped in, and the steel reinforcements inside the pillars rusted, causing the concrete to break apart — a process known as spalling. This degradation has been accelerated by seawater and typhoons in the Genkai Sea. The most notable structure, the 1916 reinforced concrete building, is now in a critical state of disrepair. In 2015, Hashima was added to the UNESCO World Heritage list as part of the sites related to the Meiji era industrial revolution. This designation reignited discussions about the use of forced labor from Korean and Chinese workers during the war. In January 2024, marking 50 years since the mine’s closure, Nagasaki held a ceremony to commemorate the island’s history, bringing together some former residents at a digital museum dedicated to Gunkanjima. Since then, the city has opened parts of the island to tourists by boat, after reinforcing walkways and observation platforms to ensure visitor safety.