Asif Bhai is right. It's one of the three water reservoirs in Dunga gali. It's a wonderful and peaceful place and one of my favorites.
Some details about the water reservoirs:
There are three ponds in Donga Gali. Water is supplied in Murree from this pond. There are two types of ponds over there, two are the square shape and one in circular shape, square shape ponds were built in 1840 and the circular shape was built in1987. All three reservoirs have the capacity to store 6250000 gallons of water collectively. At the distance of half kilometer from the pond, tanks are made in which waters from the spring is purified first, and then it is stored in the ponds.
