There are various methods of destroying human remains, depending on religious or spiritual beliefs, and upon practical necessity. Cremation is a very old and quite common custom. For some people, the act of cremation exemplifies the belief of the Christian concept of "ashes to ashes". On the other hand, in India, cremation and disposal of the bones in the river Ganges (considered by many in India, as well as Hindus around the world to be sacred) is common. Another method is sky burial, which involves placing the body of the deceased on high ground (a mountain) and leaving it for birds of prey to dispose of, as in Tibet. In some religious views, birds of prey are carriers of the soul to the heavens. Such practice may also have originated from pragmatic environmental issues, such as conditions in which the terrain (as in Tibet) is too stony or hard to dig, or in which there are few trees around to burn. As the local religion of Buddhism, in the case of Tibet, believes that the body after death is only an empty shell, there are more practical ways than burial of disposing of a body, such as leaving it for animals to consume. In some fishing or marine communities, mourners may put the body into the water, in what is known as burial at sea. Several mountain villages have a tradition of hanging the coffin in woods.