In the small village of Kadiri in Andhra Pradesh, India, there’s a tree that’s nothing short of extraordinary. Known as Thimmamma Marrimanu, it holds the title of the world’s largest banyan tree.

Some 550 years ago, so a story goes, the poet-sage Kabir was on a silt island in India's Narmada River. He was brushing his teeth with a twig. When he flung his toothbrush to the ground, up sprang a ...