Natural Bridge roots of trees. This really is cactuses, since this bridge was built almost to the root of these old trees. Using hollowed-out betel nut trunks, the tribesmen are able to direct the roots in whatever way they like. When the roots grow all the way across a river, they are allowed to return to the soil, and over time, a strong bridge is formed. It takes up to 10-15 years for a root bridge to develop, but it becomes stronger with each passing year and are known to last for centuries.