Lake Toba occupies the caldera of a supervolcano. The lake is located in the middle of the northern part of the island of Sumatra, with a surface elevation of about 900 metres. The lake is about 100 kilometres long, 30 kilometres wide, and 505 metres deep. It is the largest volcanic lake in the world.

Lake Toba is the site of a massive supervolcanic eruption that occurred 74,000 years ago, representing a climate-changing event. It is the largest-known explosive eruption on Earth in the last 25 million years. According to the Toba catastrophe theory, it had global consequences for human populations; it killed most humans living at that time and is believed to have created a population bottleneck in central east Africa and India, which affects the genetic make-up of the human worldwide population to the present.

The eruption of Toba deposited vast amounts of ash into the atmosphere and led to a volcanic winter with a worldwide decrease in temperature of 5 °C  and up to 15 °C  in higher latitudes. 

A CALDERA is a depression created after a volcano releases the majority of the contents of its magma chamber in an explosive eruption. Without any structural support below, the land around the erupting volcanic vent or vents collapses inwardly, creating the bowl-shaped caldera. Depending on their intensity and duration, volcanic eruptions can create calderas as much as 100 kilometres wide. 
www.nationalgeographic.org/news/types-calderas

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LAKE TOBA, Sumatra, is the largest lake in Indonesia and the largest volcanic lake in the world occupying the caldera of a supervolcano. (100 km by 30 km) 
We crossed it by ferry to Samosir Island  that we explored for a few days -- photos will be shared shortly. . . . No relation to Lake ManiTOBA :)
https://en.wikipedia.org/wiki/Lake_Toba

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MOUNT BROMO in East Java is an active volcano that sits in the middle of an old high plain caldera called the "Sea of Sand". We jeeped down the rim of this caldera from our mountain village homestay at Ngadas -- then took a long and bumpy ride across the plain and up the other side to view the sunrise. Then, down again to the base of smoking Mount Bromo which we had planned to climb to view the inside of its crater. We were prevented from climbing Bromo by authorities who declared that the poisonous gasses it was spewing on that day would make the climb too dangerous at that time. 
https://en.wikipedia.org/wiki/Mount_Bromo

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Bill and Sue-On Hillman Travel Adventures
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