The dolphin, named Sampal, was made to perform in the aquarium's shows, performing tricks to earn her food. At night, she was kept in a small pool.
Activists and biologists pushed for her release over four years. In 2012, the Korean High Court ordered the 10-year-old dolphin and two other dolphins in captivity be set free.
Instead of dropping the dolphins straight back into the ocean, Sampal and the others were placed in a netted sea pen to rehabilitate them before returning them to the wild.
Sampal managed to escape through a small hole in the pen -- no small feat, as dolphins avoid swimming in tight spaces -- and returned to the ocean.
She kept close to the pen for a while, but then left. Researchers later found her 60 miles away, swimming with 50 other dolphins. Those dolphins are believed to be part of her original pod.