The old cannon base No.3 which always appeares on tour guide of Tomogashima. Among the gun mountains of Tomogashima, it has the largest scale and is said to have been established with eight 28 cm grenade cannons, and four tunnels. They are connected to ground paths and underground aisles. The underground passage also leads to the ammo cabinet. You can go inside except some areas. You can feel the passing time. Also, the third rupture trail has many interest spots such as the covering part ammunition branch office and the officer dormitory trace.