The lower reaches of the southern slopes of Khasi and Jaintia hills, in Northeastern India, are humid, warm and streaked by many swift flowing rivers and mountain streams. On the slopes of this hill, among the dense undergrowth, a species of Indian Rubber tree – (Ficus Elastica) –  thrives and flourishes. These trees shoot out many secondary roots from their trunks. The trees, supported by these secondary roots, can comfortably perch itself on huge boulders along side the riverbanks or in the middle of rivers and send its roots down to the riverbed.

The ancient War-Khasi people, a tribe in Meghalaya, had noticed these qualities of this tree and had adapted it to serve their need for building bridges across rivers and streams. In order to direct the roots in the desired direction, the Khasis sliced betel nut tree trunks half in the middle for their entire length, hollowed them out and passed the thin and long tender roots through them. The roots start growing towards other end of the stream and when they are reached they are allowed to take root in the soil. Given enough time, a sturdy, living bridge is produced.

Some of these root bridges can carry fifty or more people at a time and can be over 100 feet long. These bridges take 10 to 15 years to become fully functional, and they keep growing in strength by the day. Some of these bridges are well over 500 years old.

Source: The Living Root Bridges of Cherrapunjee, India | Amusing Planet