The living bridges of Cherrapunji

One special root bridge, believed to be the only one of its kind in the world, is actually two bridges stacked one over the other and has come to be known as the "Umshiang Double-Decker Root Bridge."

 These bridges have formed from living roots over many hundreds of years and grow naturally stronger with each year. They are still used by those who live around them as passageways. 

In order to make a rubber tree's roots grow in the right direction - say, over a river - the Khasis use betel nut trunks, sliced down the middle and hollowed out, to create root-guidance systems.
The thin, tender roots of the rubber tree, prevented from fanning out by the betel nut trunks, grow straight out. When they reach the other side of the river, they're allowed to take root in the soil. Given enough time, a sturdy, living bridge is produced.

Pictures by Atlas Obscura's Wonderful Post on living root bridges.