Zhang said the team follows a principle of minimum intervention and uses traditional methods and materials, with the aim of making the repaired sections match the rest of the Great Wall.

The underwater section stands in the middle of the reservoir, so bricks and other building materials have to be taken to a dock, loaded onto a boat and unloaded at the site. Each brick weighs 15 kilograms. At most, workers can carry two at a time.

"After several rounds of carrying bricks, our backs were beginning to blister, and after working for several days, our clothes were becoming worn," a worker named Liang Wenfu said.

Engineers and workers live at the site in the middle of the reservoir.

A boat delivers supplies of food, water and daily necessities once a week. One of the engineers, Cao Huibin, once stayed on site for 45 days in a row.