Chinese authorities had evacuated nearly 200,000 people by early Saturday and warned more than 1 million others to be ready to leave quickly as a lake formed by a devastating earthquake threatened to breach its dam. Hundreds of Chinese troops have been working around the clock to drain Tangjiashan lake in Sichuan province. The lake formed above Beichuan town in the Mianyang region when a hillside plunged into a river valley during the May 12 quake that killed more than 68,000 people.