According to the statistical record great earthquakes seldom take place in Nagasaki Prefecture and the ground is comparatively stable and differential settlement on Gunkanjima is due principally to the existence of the submarine coal mine. There seem to be two main causes of differential settlement. These are as follows: A) More than two thirds of the level ground of this island is artificial ground made of 'zuri' (earth carried out of the mine), and a reinforced concrete revetment about 10 meters high surrounds the island in order to prevent landslides. Though the buildings erected before building No. 66 (built in 1948) apart from small wooden tenements and workers sheds were build upon the natural rock of the original island, buildings after No. 66 had to be built on insubstantial ground as the population increased, and these buildings therefore have a different plan and structure from the older ones. Differences include [a] change of the structural system from reinforced concrete skeleton type to reinforced concrete box frame type or reinforced concrete rigid frame type, [b] change of the foundation from piloti-like individual footings to a mat foundation, [c] addition of reinforced concrete basements except in low buildings, and so on. These measures appear to have been taken because of the low bearing capacity of the ground. However Gunkanjima is subject to violent typhoons and attendant floods in low-lying areas. Floodwater destroys embankments made of 'zuri' and carries gravel and pebbles included in the 'zuri' away into the sea. As a result, the foundations the foundations of the buildings on artificial ground became very weak. Furthermore as the revetment itself and the piers are also built on 'zuri', they have been damaged not only by rough seas but by erosion of the 'zuri. For example, the dolphin pier had been swept away twice before the present pier which is anchored deep in the rock base was built. The buildings with basements are not so badly damaged. As for the two-story brick cinema (built in 1927), the wall has cracked vertically, and eleven years after the closure of the mine, blocks between the cracks are projecting dangerously: it is only a matter of time before they collapse. The paved track along the beach has completely disintegrated and it is difficult to believe that trucks, bicycles and even roller-skaters used to use it. The concrete pool and its perimeter at the south end of the island are also totally destroyed and have become a shapeless pile of stones. B) There is another way in which the mine is responsible for differential settlement. Each mineshaft extended about 1,100 meters below the sea and the coal face was about two meters high when they abandoned one shaft and moved to another. The abandoned shaft would gradually cave in and finally disappear after about ten years, in accordance with the slow movement of the bedrock. The effect is barely perceptible on the surface, but this gradual movement of the base rock has been observed. Though the precise data is not available , this subsidence is one of the cause of the causes of the gradual enlargement of the cracks visible both on the reinforced concrete artificial ground laid over the base rock and at junctions in the structure of the buildings. This type of cracking is a major cause of damage to buildings on the rocky part of the site, while the buildings on the flat ground have been damaged by the movement of 'zuri'. Notable damage by differential settlement is visible at the expansion bridges in paved areas and on the external staircases, where expansion joints were not used because the need for them was not appreciated. In the Nikkyu company flats where beside the seaside street and often flooded, and in building No. 22, these bridges were wholly detached and leaning dangerously several years after the closing of the mine. The slope wharf behind the school which had been the only landing point after the mine was closed, became a mere mass of concrete blocks and at last sank into the sea in 1985.