Mysterious Ponds are locations from The Legend of Zelda: A Link to the Past. They are special Great Fairy Fountains, where Link can obtain upgraded equipment from the resident Great Fairy. Three Mysterious Ponds appear in the game; one behind the Waterfall of Wishing, one at the base of the Pyramid, and the Pond of Happiness at Lake Hylia. Unlike regular Great Fairy Fountains where Link's health is restored, Mysterious Ponds allow Link to either throw his equipment into the water and receive an upgraded version of that item or throw Rupees into the Pond to get certain upgrades.
The Great Fairy behind the Waterfall of Wishing can be reached after Link buys the Zora's Flippers from Zora. She can change Link's Boomerang into a Magical Boomerang and Fighter's Shield into a Red Shield, as well as fill his empty bottles with Green Potion.
The Mysterious Pond near the the base of the Pyramid can be accessed with the use of a Super Bomb. It contains a Great Fairy who is morbidly obese, and she respects the hero's honest personality, granting him the Silver Arrows that are needed to defeat Ganon. She will also turn Link's Tempered Sword into the Golden Sword. Like the Great Fairy of the Waterfall of Wishing, she will also fill any empty bottles with Green Potion.
The Pond of Happiness, which is never actually called a Mysterious Pond, is the residence of the queen of the Fairies, Venus. The beautiful Fairy enhances Link's carrying capacity for bombs or arrows when he donates 100 Rupees.