The earlier Western expedition to the mountains was led by Jan Sneep, a Dutch colonial civil servant who operated from the Sibil Valley. The expedition, which started in April 1959, mapped the terrain and collected anthropological data of the people who lived in the area. The expedition used two small Bell helicopters, but the altitudes severely limited their effectiveness and one of them crashed, forcing the expedition to rely more on traditional man power. Climbers from the expedition reached the peak of Puncak Mandala on 9 September.