Mysterious Ponds are locations featured within A Link to the Past. There are two Mysterious Pond locations, one at the Waterfall of Wishing within the Light World and another at the Pyramid of Power within the Dark World. Link can toss items within the pond, summoning a Great Fairy in the process. If Link tosses specific items within the pond and is honest to the Great Fairy, Link may get an upgraded version of the particular item.
At the northwest portion of the Waterfall of Wishing, there is a waterfall that Link can swim inside. Link needs to have the Flippers in order to access this Mysterious Pond. Inside, Link is able to toss an item into the Mysterious Pond, causing the Great Fairy to appear. The Waterfall of Wishing Fairy is a friend of Venus and rewards Link if he is honest about whether he tosses an item into the pond.
There are three items that Link can acquire here. If Link throws in the Boomerang, he gets the Magical Boomerang in return. If Link throws in his Fighter's Shield, he gets the Fire Shield in return. If Link throws in a Magic Bottle, he gets a Magic Bottle filled with Green Potion in return.
The other pond is where the Cursed Fairy lives, and located inside the Pyramid of Power. It is only accessible after defeating the first two Dark World dungeons: Ice Palace and Misery Mire. After that, Link must go to the Bomb Shop and receive the Super Bomb. Within the allotted time limit, Link must reach the pyramid and the wall on the second level.
It is completely necessary to find the Dark World pond, as tossing the Bow into it is the only way for Link to obtain the Silver Arrows that are necessary to defeat Ganon. Link can also toss the Tempered Sword in to receive the Golden Sword.
The Mysterious Pond is located at the northwest part of the Waterfall of Wishing.
Link tossing a Boomerang into the Mysterious Pond.