Deep within Nelson Lakes National Park, in New Zealand’s South Island, lies a magical blue-violet lake. Surrounded by steep alpine forests and spring-fed from the glacial waters of Lake Constance, it is small but not insignificant.

It was first discovered by Ngāti Apa, a Māori iwi, or tribe, who named it Rotomairewhenua, meaning “the lake of peaceful lands.” It became a sacred place where they cleansed the bones of the dead, believing this would secure the spirit a safe journey to the ancestral Māori homeland of Hawaiki.

In more recent history, hikers passing through the national park have commented on the lake’s extraordinary color and ethereal energy, but it was only around a decade ago that scientists discovered that the water of the sub-alpine lake had “exceptional optical purity” with a visibility of between 70 and 80 meters. That’s in line with that of pure water, which they said made it “the most visually clear freshwater yet reported.”

The title of “clearest lake in the world” and photos of the stunning landscape have since been shared widely on social media, making the lake a popular tourist destination between December and March (during New Zealand’s summer). But conservationists and the Ngāti Apa now fear that this increased popularity could threaten the purity of the lake.

Their biggest concern is the spread of lindavia, a microscopic alga known colloquially as “lake snow” or “lake snot” for the slime it creates that hangs just below the surface of the water. The alga is already present downstream of Rotomairewhenua (also known as Blue Lake) in lakes Rotoiti, Rotoroa and Tennyson, and is at risk of being carried up the trail on hikers’ boots or in their water bottles.