n the depths of northeastern India, in one of the wettest places on earth, bridges aren't built - they're grown. The southern Khasi and Jaintia hills are humid and warm, crisscrossed by swift-flowing rivers and mountain streams. On the slopes of these hills, a species of Indian rubber tree with an incredibly strong root system thrives and flourishes. The Ficus elastica produces a series of secondary roots from higher up its trunk and can comfortably perch atop huge boulders along the riverbanks, or even in the middle of the rivers themselves. The War-Khasis, a tribe in Meghalaya, long ago noticed...