Lake Segara Anak, Volcanic crater lake in Rinjani National Park, Indonesia

Lake Segara Anak fills the caldera left by the 1257 eruption of Mount Samalas, beside Mount Rinjani on Lombok. The lake measures 7.5 by 6.0 kilometres, reaches 230 metres deep and lies at 2,004 metres above sea level. Gunung Barujari, a volcanic cone at its eastern end, gives the water its crescent shape.

Segara Anak fills the caldera left by the 1257 eruption of Mount Samalas, whose estimated pre-eruption height was 4,200 metres. A 2013 study estimated the eruption ejected up to 42 km3 of dense rock, a Volcanic Explosivity Index 7 event. It may have lowered global temperatures for several years and been a factor in the Little Ice Age.

The name Segara Anak means "child of the sea" in Sasak, for the blue water's resemblance to the sea. Trekkers descending to the shore leave offerings for the deity at the lakebed, and many make for Goa Susu, a natural sauna cave where they bathe at night and where Hindus place offerings. A myth places the palace of Dewi Anjani, ruler of Mount Rinjani, in the middle of the lake.

Trekkers usually reach the lake on multi-day routes from Sembalun Lawang or via the Torean forest trail, where Goa Susu cave serves as a natural sauna and overnight stop. The crater rim climbs to 3,726 m at Rinjani's summit, so plan for cold nights and steep, loose ground.

The lake holds millions of tilapia and carp that were introduced by the West Nusa Tenggara provincial government in 1985, after volcanologists from the Geological Society of London surveyed the water in 1969 and recommended stocking it with fish. Some local people now make a living from fishing here.