Gunung Wilis is a solitary volcanic massif surrounded by low-elevation plains on all but its southern side. It was formed during three episodes dating back to the mid Pleistocene. Following destruction of the 2nd edifice, the most recent cone grew during the Holocene. No confirmed historical eruptions are known, although there was a report of an eruption in 1641 CE, the same year as a major eruption of nearby Kelut. Fumaroles and mud pots occur near Lake Ngebel on the lower western flank. (Source Smithsonian GVP 2021).