This is a large circular cistern neatly scooped out of a large boulder located in front of the Trimurti Cave. Its presence is not very obvious , as the outer portion of the boulder is left unfinished and looks like a boulder in the natural environment. There are a few steps by the side of the stone Cistern access its view from the top. Probably this was used as storage of water for the ceremonial purpose.
Regarding its name Gopis (or Gopikas) refer to the group of cow herding girls famously associated with the Lord Krishna in Hindu mythology.
What is curious is the perfection with which the circular shape is achieved, especially with those rudimentary chiseling tools available those days. But then that goes true for much of the carving works in Mahabalipuram.
Mahabalipuram at a distance about 60km south of Chennai is a UNESCO World Heritage Site in India.