In the Northeast Indian state of Meghalaya, a strange sort of bridge exists. These "living root bridges" are a form of tree shaping that are made from the aerial roots of rubber trees.
Though these bridges are created with the guidance of the Khasi and Jaintia people, they are strong enough to support about 50 people at a time. The roots grow and strengthen over time. It takes approximately 20 years for the bridge to be strong enough to hold people.
A root bridge can last for hundreds of years as long as the tree remains healthy. The bridge will naturally self-renew and self-strengthen as its roots grow thicker. Unlike an artificial structure, these living bridges are able to withstand the high level of soil erosion that occurs by monsoon rains. Living material is also much more resistant to destruction by termites, and animals as it is able to renew itself if damage is inflicted.
In order to make the rubber tree's roots grow in the right direction, several different strategies are used. Sometimes the roots of the trees are pulled together, tied, and twisted by hand for them to merge together over time. This is the most common method.
Less common ways include creating scaffolds out of wood and bamboo, and then training the young roots of the rubber tree to go out across them.
In some places, betel nut trunks are sliced down the middle and hollowed out to create root-guidance systems. This gives the roots an advantage, as they get nutrients from the trunk as it decays.