From Zelda Legends Wiki
Located just a bit to the south of a huge waterfall and next to a teleporter there is a small keyhole which looks like it is not important at first. After retrieving the Angler Key from Yarna Desert you use it here to gain entrance to the Level 4 Dungeon: Angler's Tunnel.
|Japanese||アングラーのカギあな (ANGURĀ no KAGIona)|
The Japanese and English versions correspond with each other, but the French "Fish Lock" and German "Miraculous Keyhole" are somewhat different from the original.