One of the oldest step wells in Delhi, was constructed by Sultan Iltutmish, somewhere around 1220s for Qutub Sahab, Bakhtiyar Kaki who was a Sufi saint of the Christi order.
The water is high in sulphur content and is reputed to have medicinal qualities. A green layer of algae covers the top, but if you part the water gently, it is crystal clear even today. The depth is quite misleading. It's at least 40 feet from the water as it stands today. That's seriously deep …