Young orangutan Kevin takes a nap at Nyaru Menteng

Drøscher Nielsen sought the advice of Dr Willie Smits of the Borneo Orangutan Survival (BOS) Foundation about the possibility of creating a new project in Central Kalimantan to deal with the swelling numbers of orphaned orangutans. Dr Smits agreed to help, and with the financial backing of the Gibbon Foundation and BOS Indonesia, Lone founded the Nyaru Menteng Orangutan Reintroduction Project in 1998.[3] She was able to build the facility under an agreement with the Indonesian Ministry of Forestry, and Nyaru Menteng officially opened its doors to the first dozen orangutans in 1999.

The sanctuary was designed to hold up to 100 orphaned orangutans while they go through rehabilitation. In addition to quarantine cages, medical clinic, and nursery, the sanctuary had a large area of forest in which orangutans could learn the skills needed to live in the wild.[4] Nyaru Menteng quickly became the largest primate rescue project in the world, with over 600 orphaned and displaced orangutans in its care in 2009[5] and a staff of 200.[6]

Many of these orangutans are only weeks old when they arrive, and all of them are psychologically traumatized.[7] The sanctuary not only saves the mostly orphaned baby orangutans from the local farmers and illegal pet-traders, but has developed a process for their gradual re-introduction to the remaining Borneo rainforest.

Babies and young orangutans brought to the centre are cared for 24 hours a day by a team of "babysitters". As they grow and learn they are then taken to "forest school" where they learn, still with staff present, to climb trees and survive in the forest. At the age of about eight years, they are relocated in groups of around 25 to a neighbouring island for the first stage in their release.[8][9]

At the present, Drøscher Nielsen lives near the Nyaru Menteng Rescue Center, Kalimantan, Borneo, managing a specialized clinic of veterinarians and paramedics as well as a workforce of local Indonesians who work as babysitters caring for the orphaned orangutans in the center.