A herd of sheep move to an island on the frozen Puma Yumco Lake, Southwest China's Tibet autonomous region, Feb 9, 2017.[Photo/Xinhua]

Migrating to pastures new in the depths of winter is tough, but traveling across a frozen lake with thousands of sheep in tow poses extra challenges, and that is exactly what is happening at Puma Yumco Lake, one of the world's highest lakes.

It is the coldest point of the year in Tibet, and time for the annual migration of herds of sheep across the icy lake, a dangerous and exhilarating ancient practice.

Transhumance involves moving livestock between fixed summer and winter pastures, and here it means thousands of sheep being taken across frozen water to two islands on Puma Yumco Lake.

Such herding practices have lasted for a thousand years at Dowa village in Nakartse county, Tibet Autonomous Region. The village is 5,070 meters above sea level. The summer is short here, and herders have to find grass for their animals.

Herders spend a month with the animals on the islands, where grass grows to waist height. The sheep are better fed there, and give birth to lambs during the month.

On Feb 9, Kunsang Cering, 47, a herder, led the first trip across the lake this winter.