Samosir is a large island sitting within the caldera of Lake Toba, itself a supervolcanic lake in the highlands of North Sumatra. The combination — an island within a lake within a volcanic caldera — is geologically remarkable, and the scenery is extraordinary: steep green cliffs dropping to dark water, with the island's plateau rising in the centre. The eruption that created the caldera around 74,000 years ago was one of the largest in Earth's history; what you are looking at when you gaze across the lake is the vast hollow it left behind.
The island is the heartland of the Batak Toba people, and their culture is strongly present here. Traditional Batak houses, with their distinctive boat-shaped rooflines and elaborately carved facades in red, black, and white, stand in the villages along the shore. The village of Tomok has a royal cemetery with ancient stone sarcophagi, and Simanindo on the northern tip has a well-preserved traditional house complex with performances of Batak music and dance.
Most travellers base themselves at Tuk Tuk, a small peninsula on the east coast with guesthouses, restaurants, and easy swimming from the lakeshore. On the plateau in the island's interior sits Lake Sidihoni — a lake on an island in a lake on an island, adding one more recursive layer to an already improbable geography. The island is big enough to explore properly by scooter, which can be hired locally; the road that circuits the perimeter passes through Batak villages and gives continuous views over the caldera rim.