Use stealth as there are always muggles around this famous spot.
Legend has it that Emperor Le Loi obtained a magical sword from the Dragon King to fight against oppressors. The sword came from Hoan Kiem lake. After the successful battle against the invaders, Emperor Le Loi visited the beautiful lake. He was boating through the still waters when an enormous golden turtle surfaced. The turtle explained that he was sent by his master, the Dragon King, to retrieve the sword from Le Loi. Loi returned the sword to the turtle, who swam back beneath the jade waters. Loi then named the area Hoan Kiem lake, meaning “Lake of the Returned Sword” to commemorate the spectacular event.
From the cache you have a nice view of Jade Island, on which the Temple of the Jade Mountain (Ngoc Son Temple) stands. Jade Island is connected to the shore by the wooden red-painted The Huc Bridge/Cầu Thê Húc (meaning Morning Sunlight Bridge).
Congrats to Ben.ny14 for being first to find.