The summit of Keli Mutu (1620m) is a startling lunar landscape, with, to the east, two vast pools separated by a narrow ridge. The waters of one are a currently luminescent green that seems to be heading for bright-yellow, the other was, a few years ago, a vibrant turquoise, and is now deep magenta. A few hundred metres to the west, in a deep depression, is a pure-black lake. The colours of the lakes are apparently due to the levels of certain minerals that dissolve in them. As the waters erode the caldera they lie in, they uncover bands of different compounds and, as the levels of these compounds are in constant flux, so are the colours. In the 1960s, the lakes were red, white and blue, and locals predict that within years they will have returned to these hues.

Every morning at around 4am, an open-sided truck takes travellers from Moni up to Keli Mutu, returning at about 7am (Rp10,000 one way, minimum five people, plus Rp1000 park fee). The best view is from the south crater rim, looking north over the two sister lakes; the trails that run around other rims are extremely dangerous - tourists have disappeared up here. A much nicer alternative to returning with the truck is to walk back down to Moni, which takes about three hours, with rolling grassy meadows flanking extinct volcanic hills, and views all the way to the sea. Practically the whole walk is downhill, but always bring water and wear good boots. A shortcut by the PHPA post cuts off a good 4km from the road route, takes you through some charming local villages and past the waterfall ( air terjun) less than 1km from central Moni, which is a great spot for a dip after what can be a very hot walk. A little further down is a hot spring, the perfect place to soak weary feet.