This is the largest of the mine tunnels that have been discovered and surveyed, with a total length of about 900 meters. It includes a huge open space that measures 20 meters high, 30 meters deep, and 15 meters wide. You can see the remains of tunnels thought to have been hand-dug from the Edo period, as well as the traces of machine mining that widened the tunnels in the Meiji era. Guided tours are available by reservation for a limited period from March to November.