Defeat the Lanmola in Yarna Desert as normal, but make sure its head is far away from the pit in the center of the screen. If you grab the key at the right distance, you can walk away from the hole and not fall down it. To duplicate the key, you must be the exact distance that would put you a pixel or so away from falling down after the celebratory music. If it works, you'll see the key drop after the text box goes away. Fall down or enter the cave and you'll see the Angler Key still in the cave!
Submitted by Artemis251.