Lake Kutubu lies east of the Kikori River, into which it drains, and about 50 kilometers southwest of Mendi, the provincial capital. Unlike most bodies of water in this mountainous country, which form behind glacial moraines or in volcanic craters, Kutubu occupies a natural depression in the highland terrain. Its waters are fed primarily by streams originating from underground sources, giving the lake a distinctive clarity. A few small islands dot the surface, the largest being Wasemi in the northern section. The Kutubuan languages, part of the Trans-New Guinea family, are spoken in the surrounding communities, connecting the lake's human inhabitants to a linguistic tradition that stretches across much of the island.

The lake's isolation has produced a remarkable concentration of endemic fish. At least thirteen species are found here and nowhere else. The Kutubu tandan, the Kutubu hardyhead, Adamson's grunter, and no fewer than seven species of mogurnda gudgeon are among them. Two species of Glossogobius goby remain so poorly studied that scientists have not yet given them formal names, referring to them only as "new sp. 8" and "new sp. 12." This biological richness earned the lake designation as a Wetland of International Significance under the Ramsar Convention, and the World Wildlife Fund undertook a catchment management study intended to serve as a model for watershed protection across Papua New Guinea. The lake is also included in the tentatively listed Kikori River Basin and Great Papuan Plateau UNESCO World Heritage Site.

Kutubu's isolation protected it for millennia. Then the oil industry arrived. Petroleum development in the surrounding highlands has supported the local economy but also triggered rapid in-migration to the area. The consequences have been predictable and severe: pollution, forest destruction along the lake's margins, and overfishing that threatens the very species that make Kutubu ecologically irreplaceable. A proposed gas pipeline and accompanying road threaten to accelerate these pressures further. The catchment management plan commissioned through the Asian Development Bank aimed to establish protective frameworks before the damage became irreversible, though the timeline for that protection has consistently slipped.

The tension at Lake Kutubu is the tension of modern Papua New Guinea in miniature. The highlands hold enormous mineral and energy wealth, and the country desperately needs the revenue that extraction provides. Yet the ecosystems that make these highlands globally significant, the endemic species, the intact watersheds, the cultural landscapes of the Kutubuan-speaking communities, cannot survive unmanaged development. Kutubu sits at the intersection of these forces, its clear waters reflecting both the forested slopes above and the industrial infrastructure creeping closer. For now, the rainbowfish still catches the light. Whether it will continue to do so depends on choices being made not at the lakeside but in boardrooms and government offices in Port Moresby and beyond.