The lake and island were formed after the eruption of a super volcano some 75 000 years ago. The island was originally connected to the surrounding caldera wall by a small isthmus, which was cut through to aid navigation.
At 630km², Samosir is also notable as being the largest island within an island, and the fifth largest lake island in the world. It also has 2 named lakes, Lake Sidihoni and Lake Aek Natonang. Across the lake on the east from the island lies Uluan Peninsula.
The island is actually linked to the mainland of Sumatra on its western part by a narrow land connecting the town of Pangururan on Samosir and Tele on the mainland Sumatra. Tele is one of the place that offer the best view of Lake Toba and Samosir Island.
Samosir is a popular tourist destination due to the vistas it offers. The tourist resorts are concentrated in the Tuktuk area. The island is the centre of the Batak culture and many of this people's artifacts remain on the island. Almost all of the tourist accommodation are concentrated on the small town of Tuktuk, which is one hour ferry ride across the lake from the town of Parapat.