Located 100km southwest of Lhasa and at an altitude of 4,440m, Yamdrok Yum Tso Lake is one of the four holy lakes in Tibet.     Covering an area of 638 km², it is fan-shaped, spreading to the south but narrowing up to the north. It freezes in winter.     Like mountains, lakes are considered sacred by Tibetan people, the principle being that they are the dwelling places of protective deities and therefore invested with special spiritual powers.