"Magic, darling," Rarity answered. "It's an Old Mare's Tale, but they say that when Nightmare Moon fought with Princess Celestia, one of her spells during the battle hit a tiny pond where the Two Ruling Sisters used to feed the local critters, and for a thousand years the spell has been rapidly freezing and unfreezing that tiny pond until it became the lake we see now." She explained as she gestured toward the water. 