The Hashima coal mine of the Mitsubishi Mining Co. is located at the semi-artificial tiny Hashima Island, well known as "Warship Island", about 20 km to the south-west of the Nagasaki Harbour.
The mine was put into operation in 1887 and has yielded highest class coking coal ever since. Nevertheless, the mining condition was becoming unfavorable in recent years and new coal fields have been actively searched in this area.
Since 1953, many geologists and their assistants collected large number of rock samples from the sea-bottom by means of the specially designed dredger backet. They dived with aqualung to observe directly the outcrops at the sea bottom, and examined heavy mineral frequencies of the rock samples collected. Intensive geophysical prospecting, such as marine seismic prospecting by reflection method with floating cables and pressure sensitive geophones, and sonic prospecting was also introduced, in 1957.
Thus, the detailed geological maps could be constructed, and the underground geological structure in this off-shore area was revealed. Several coal seams in the Mitsuse area could be traced at the sea bottom exactly, and the coal reserves were clarified.
The Mitsuse area has been mined, since Oct. 1965, 14 months after the previous working area was closed owing to a mining accident. The Mitsuse area produces monthly more than 30, 000 tons of high class coking coal at present.
In the Hashima area, there is a large reverse fault, named the Hashima-oki fault. Since the depth of the coal seams beyond this fault is rather shallow and there is a large quantity of coal reserves, this huge area is much promising as the next working area.
