Permafrost. Thin air. Extreme cold.

At 4,800 meters on the Qinghai-Xizang Plateau, we finally understood why the Qinghai-Xizang Railway is called a miracle. For 20 years, this "heavenly road" has crossed snow-capped mountains and uninhabited lands. Yet its greatest challenge may lie beneath the tracks: invisible permafrost that can shift when it thaws.

How was the world's highest railway built on such fragile ground? Follow Liz across the "roof of the world" to find out.