Gunung Tahan

West Malaysia's highest mountain, Gunung Tahan, is a rocky island in a lush sea. The virgin rain forest on and around this 7,175-foot peak forms one of the world's oldest undisturbed ecosystems; indigenous flora and fauna have coexisted here for 130million years.

Along your trek to the summit, you'll encounter such unique species as Grammatophyllum speciosum, the world's largest orchid, carnivorous pitcher plants, and the Tualang tree, one of the world's tallest. Also making their home here are numerous mammals, including tigers, leopards, elephants, and the endangered Sumatran rhino, though your chances of seeing these shy beasts are slight. But take heart: It will be nigh impossible not to spot the countless colorful birds and butterflies sharing the trail with you.

This multiday excursion begins with a three-hour boat ride to Taman Negara National Park headquarters, where the route begins. The terrain is wet and demanding: You'll face seven river crossings in a day and rope climbs along a rocky ridge crest (though no technical climbing skills are necessary). Take a guide, who will help you differentiate between the pretty and the poisonous, and be aware that you're not the only people in the forest. The Orang Uli ("native people" in Malaysian) still make their home here. They are not hostile, and you probably won't see them, but chances are they'll see you. The trip typically takes ten days round-trip.