The group of soldiers were carrying out an exercise linked to forest fires near Morgins lake in the Portes du Soleil region when the pump used to extract water from the lake had a technical problem and started leaking hydraulic fluid, according to Valais newspaper Le Nouvelliste.
In total 50 litres of oil ended up in the lake, which is protected due to its status as a national amphibian breeding site.
Firefighters and police were called to the scene to help with the clean up operation, which took four hours.
Speaking to the paper, an army spokesperson said: “There’s no problem for the environment because the oil stayed on the surface and could be removed. Everything went back to normal.”
According to the Swiss federal institute for forest, snow and landscape research, amphibians are one of the most threatened groups of animals in Switzerland, with 70 percent of species on the endangered list, so preserving their breeding sites is essential.