A smuggler's tunnel is a secret passageway that leads from Fisherton to the shores of Monster World. Like many folk tales and throughout history, smuggler's tunnels have been used by criminals to bring contraband to countries at the dead of night and steal them into town through passageways concealed under people's homes. One particular tunnel used by Shion and Baila was a dark, steep staircase, only lit by the burning town above them. Contrasting with real world smuggler tunnels, this one was guarded by a giant worm. Shion made light work of it by slashing it with his sword and using magic. The tunnel eventually widened and reached the shore, where Shion could see the wagons containing innocent villagers.
As opposed to the actual purpose of smuggling tunnels, the shore entrance of the cave was extremely visible to the naked eye and could easily be accessed by anyone by walking along the beach. In turn, this meant that anyone could just walk into the connecting house (if they could get past the worm, that is).