This is Lake Toba in North Sumatra which is the caldera of the Toba supervolcano which measures about 100 x 30 km in size. It's only noticeable from a satellite image ... https://en.wikipedia.org/wiki/Lake_Toba
When the Toba Supervolcano erupted about 70,000 years ago, geologists and archaeologists suggest that it disrupted human evolution for several millennia.