THE forests stand like a fortress on the border between Kalimantan and the East Malaysian state of Sarawak. The trees, holding close to each other and reaching for the sky, and the clumps of bamboo and thick underbrush below, seem impenetrable. This is Kayan Mentarang National Park, the last natural fortress left in the area.

From the nearest border village in the East Kalimantan district of Tarakan one has to sail down the river for over 10 h