Segara Anak

Segara Anak is the crater lake filling the caldera of Mount Rinjani on Lombok, and at 2,004 metres above sea level it is Indonesia's second-highest caldera lake sitting beside an active volcano. The caldera itself is the scar of one of the most violent eruptions in recorded history: in 1257, the volcano then known as Mount Samalas collapsed in an eruption that ejected an estimated 10 cubic miles of dense-rock-equivalent material, an event some climate researchers link to a documented period of global cooling in the centuries that followed. What remains today is a lake 7.5 by 6.0 kilometres across, covering 11.3 square kilometres and reaching 230 metres at its deepest point. Its name, Segara Anak, means child of the sea in reference to the water's deep blue, ocean-like colour, an image well known enough in Indonesia that the lake appears on the 10,000-rupiah banknote. At the lake's eastern edge, the young volcanic cone of Gunung Barujari rises to 2,376 metres, an active vent that gives the lake its distinctive crescent shape by crowding against one side of the caldera. Because magma still sits close beneath the caldera floor, the lake runs warm for its altitude: recorded temperatures of 20 to 22 degrees Celsius are some five to seven degrees higher than would be expected at 2,000 metres, warm enough for swimming despite the surrounding cold mountain air. The lake was originally fishless, but a stocking programme begun in 1985 introduced tilapia and carp, which bred rapidly and turned Segara Anak into a working fishery as well as a trekking destination; anglers and campers now share its shores with hikers. Reaching the lake requires a multi-day trek within Gunung Rinjani National Park, most commonly via the Sembalun or Senaru trailheads, with an overnight camp on the crater rim before the steep descent to the water, a serious undertaking reserved for prepared, guided hikers rather than a casual day trip.