To save their commuting time, the couple established a temporary home in a cave near the paintings. They have some daily necessities, including a bottle of insecticide, two pots, a chopping board, a kitchen knife and a simple bed made of thin wooden planks.

"The insecticide is to keep mosquitoes out and snakes," Zhang says.

The couple have placed their cooking oil, salt and rice in a plastic bucket and hung it on the wall of the cave in case rats appear.

They also use a small piece of land in front of the cave to grow chilies. They fetch fresh water from a cave at the bottom of the cliff.

"The cave is cool in summer and warm in winter," Zhang says.

Having spent six years guarding the paintings, Zhang has lost count of how many nights he has spent in the cave. But he is getting familiar with the paintings.

"I can tell you the exact location of each and every painting," he says.