Wikipedia
Moriro
Moriro ain Mangermachh is a story of personal valor and inventive technique by which the hero Moriro(the fisherman-seeker) killed a sea-monster Mangermachh ( Crocodile) by diving deep in an especially built Iron-Cage.
The brave fisherman, who went into the sea to bring back his dead 6 brothers who had been swallowed one after one by the devouring whale.