That's Rani-ki-Vav, one of UNESCO's World Heritage Sites in India. What is that?

Rani-ki-Vav (the Queen’s Stepwell) is situated in the town of Patan in Gujarat, India. Stepwells are a distinctive form of subterranean water resource and storage systems on the Indian subcontinent, and have been constructed since the third millennium BC. Rani-ki-Vav was built with an inverted temple, seven levels of stairs, and holds more than 500 principal sculptures.