The legend of Urashima Tarō

The legend of Urashima Tarō (浦島太郎?) is a Japanese legend about a fisherman who rescues a turtle and is rewarded for this with a visit to the Palace of the Dragon God (or Ryūgū-jō) under the sea.
One fine day a young fisherman named Urashima Tarō was fishing when he noticed a small turtle being tormented by some children. Tarō saved it and let it go back to the sea. The day after a huge turtle approached him and told him that the small turtle he had saved was the daughter of the Emperor of the Sea, who wanted to see him to thank him. The turtle magically gave Tarō gills and brought him to the bottom of the sea, to the Palace of the Dragon God (Ryūgū-jō). There he met the Emperor and the small turtle, who was now a lovely princess, Otohime.
Urashima Tarō illustration by Edmund Dulac
Tarō stayed there with her for a few days, then he was caught by the desire to go back to his village and see his aging mother, so he asked her permission to leave. The princess said she was sorry to see him go, but wished him well and gave him a mysterious box which she told him never to open, for whatever reason. Tarō grabbed the box, jumped on the back of the same turtle that had brought him to the Palace, and soon was home.
But everything had changed. His home was gone, his mother had vanished, the people he knew were nowhere to be seen. He asked if anybody knew a man called Urashima Tarō. They answered that they had heard someone of that name had vanished at sea long ago. He discovered that 300 years had passed since the day he had left for the bottom of the sea. Struck by grief, he absent-mindedly opened the box the princess had given him. Out of it came a cloud of white smoke. He suddenly aged, his beard grew long and white, and his back bent. He was now a very old man. And from the sea came the sad, sweet voice of the princess: "I told you not to open that box. In it was your old age ..."
As always with folklore, there are many different versions of this extremely famous story. In one, for example, after he turned into an old man he took the body of a crane, in another he ate a magic pill that gave him the ability to breathe underwater. In another version, he is swept away by a storm before he can rescue the turtle. Also, there is a version in which he dies in the process of aging (his body turns into dust), as no one can live 300 years.