His wife retired in 1992 and he retired in 2001, at age 60. They had earned a monthly salary of 6,000 yuan ($880) together, and their children scrambled to Xinjiang to try to take them back to their home province, Shandong. But Wei and Liu refused to leave Sarbulak.

"I'd feel uneasy if I couldn't walk along the border every day," Wei told his children.

Wei had a stroke in 2012 and was hospitalized, but nobody could persuade him to rest for long and he returned to Sarbulak as soon as he felt slightly better. "I thought this time he would finally move back," said former comrade Cao Guiying. "I never expected that he would go back to Sarbulak again just to keep guarding the homeland."

Wei turned 78 this year, hair silvered and face wrinkled. He and his wife still watch the sun rise and set every day from their home near the No 173 boundary marker between China and Kazakhstan.