Legend has it that before the tunnel (Ao no Doumon) was constructed, people had to scale the cliffs through which the tunnel now passes to reach an important local shrine. The route was extremely hazardous, and people regularly fell to their deaths. During the Edo period, a Buddhist monk named Zenkai, who had committed murder earlier in his life, decided to build a safe passage for worshipers to atone for his crime. Starting at the age of 49, he spent 30 years of his life digging the 185-metre tunnel by hand, using only a hammer and chisel.