Staring close up at the colossal Sopara Stupa, set in a tiny palm-fringed Archaeological Survey of India garden, it is hard not to be amazed. Its bricks were of such superior quality that this structure has survived for millennia on the outskirts of Mumbai, just a few kilometres from the Nalasopara suburban train station. According to estimates the stupa may be an astonishing 2,200 years old.