Lake Toba, the largest lake in East Asia and the largest volcanic lake in the world is found in North Sumatera, Indonesia. The crater lake was formed because of the massive supervolcanic eruption of Toba.
But legend has it that a princess turned fish turned human was behind it. Below is the story/legend of the Lake Toba.
It is said that a long time ago, a poor young man lived alone in this area. This orphan made his living by farming and fishing. One day he captured a peculiar fish. Perplexed with the unusual shape, he touched it, and the fish became a beautiful princess. The princess told him that she was cursed for breaking a rule made by the gods. The gods decreed that the fish should take shape similar to a creature that touched her the first time; hence, the fish became human.
He proposed to her and she agreed on one condition, that he’d never divulge the secret of her origin. He agreed to this and they got married. Later on they had a son, and were happy even though they still had to work hard.
One day the boy was so hungry that he ate all of the food in the house, including his parents’ share. His father became so angry that he cursed his son, betraying his true parentage. Suddenly the wife and son vanished. The man regretted what he had done but there was no turning back. A spring came out from where the son and wife disappeared, which after some time became Lake Toba. ( Source: http://www.indonesia.travel)