Hundreds of thousands of national treasures housed in the Palace Museum, also known as the Forbidden City, will soon find a more protective home underground.

Construction of a third underground warehouse for cultural relics began at the museum on Tuesday.

The Forbidden City was China's imperial palace from 1420 to 1911－the Ming and Qing dynasties－and houses 1.86 million cultural relics.

Shan Jixiang, the museum's director, said about 970,000 relics are housed in underground storage areas, and the rest are in various places around the site.

"Due to outdated technology, all the artifacts underground are treated the same and stored at the same temperature and humidity," he said. "We have to create tailored conditions to protect different type of artifacts. A new warehouse with a better environment is therefore needed."

The first underground warehouse at the Palace Museum was built between 1987 and 1991 and covers 4,818 square meters. The second phase of construction lasted from 1993 to 1997 and added another 15,970 square meters.

Special cases to protect artifacts from earthquakes have been introduced into the two existing warehouses in recent years, eating up precious space.

Shan said the new warehouse, which will have 8,285 square meters of floor space and connect the two older warehouses, will be equipped with better conservation facilities to keep artifacts away from threats like leaking water.

The new storage area will be finished by 2020, the 600th birthday of the Forbidden City, Shan said.

He said it will also ensure the safety of many ancient artifacts by removing them from storage in the aboveground wooden structures, which are subject to fires.

A new underground channel will also be built connecting the new warehouse and the so-called cultural relics hospital in the western part of the museum.

The "hospital", one of the world's largest conservation centers of cultural relics at a museum, was built in 2016. Shan said the tunnel will enable the movement and daily maintenance of relics and avoid disturbing other parts of the museum.

"We will have bigger exhibition space as well, when these warehouses are cleared," Shan said.

For example, Nandaku, or the southern grand warehouse－the largest in the Forbidden City during imperial times－will be opened to visitors later this year as a permanent gallery displaying furniture from the Ming (1368-1644) and Qing (1644-1911) dynasties.