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Because of the huge eruption of Mount Toba some foreign researchers conducted research on the causes of Lake Toba. It is estimated that the eruption of Mount Toba is one of the most powerful eruptions ever since the total material released reaches 2,800 km3 - 2,000 km3 of Ignimbrit that flows over the ground, and about 800 km3 which falls as ash to the west. The pyroclastic flow from the eruption destroys an area of 20,000 km2, with ash deposits as thick as 600 m with the main crater. This leads to the formation of nature which is water which is now known as Lake Toba and makes upward by magma that has not yet come out into an island in the middle of the lake known as Samosir Island.

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Many of the folklore that tells the formation of Lake Toba and Samosir Island, one of them as follows: Once there is a young man named Toba and Toba's daily activities are fishing. One day Toba went fishing in the river, Toba has been fishing all day but he did not get a fish. With a sense of despair because the sun was about to set, Toba dumped his hook into the deepest water, suddenly he gasped as his hook got a huge gold fish. With happiness, he lifted the fish and went to go home.

But Toba was startled to hear the sweet voice of the gold fish "Do not take me to your house for you to cook, but put me in a little hut in the fields, and come there after three days." Toba surprised to see the fish can speak, and he followed the words of the gold fish.

After three days of waiting Toba, he went to the hut. Toba gasped in surprise that he did not find a golden fish in the hut, but he found a very beautiful girl with long hair like an angel who descended from the seventh heaven. Feeling shocked and amazed at the beauty of the girl Toba asked the girl "Who are you and why are you here? ", The girl replied softly" I am named Nauli, the princess of the ruler in the water. I am commanded by the Gods for man ".

As a young man Toba seduced the girl to want to be his life companion. The girl said "I want to be your life companion but with a condition of a covenant. Whatever happens in our household, you can not say that I am the incarnation of a fish and if you break the promise, I will return to being a fish and live again in the water. " Toba agreed with the agreement and they set up a household.