Braj has 12 large forests (vans) and 24 smaller forests (upavans), a total of 137 groves, 6 hills, innumerable kunds, and is inhabited by over 600 hamlets. Once an ecologically rich zone, much of its flora and fauna is lost. Only 3 groves remain out of 137, the western hills were illegally mined and the kunds, said to number over 1000, are drying up and are polluted. The holy river Yamuna is heavily polluted with sewage, its waters unfit for ritual bathing. Yet pilgrimage is flourishing and devotees from all over India continue to arrive in Braj in ever-increasing numbers to do parikrama (circumambulation), also known as ban yatra. The longest of these is the chaurasi kos (84 kos or 168 miles) parikrama that takes the pilgrims on the circumambulatory tour of the main forests and water bodies as well as the holy cities of Mathura and Vrindavan. The cultural heritage of Braj is extraordinarily rich in mythology and folklore sustained by natural and built-up sites reclaimed repeatedly over centuries.