Agam Kuan -- the "unfathomable well" -- is a brick-lined shaft about thirty metres deep in the Gulzarbagh area of east Patna. The lower section is cut into the rock and was probably dug in the Mauryan period; the upper section has been rebuilt over the centuries. It is one of the few Mauryan-era structures still in use as it was originally intended -- pilgrims still draw water from it.
A small temple to Shitala Mata, the goddess of smallpox, stands beside the well. The site is a short autorickshaw ride from Patna City station and combines well with a visit to Takht Sri Patna Sahib. Easily missed -- ask for directions.