Three connected entrances of the tunnel were found at a depth of one metre. The underground passage was vaulted solidly in classical architecture without steel framework.

This was not the first time such tunnel traces have been discovered at Chu Van An School, said Do Van Huyen, a member of the school’s administration department. In 2005, construction workers also uncovered similar relics at the site but they filled them up for building works.

According to some elderly people living near the school, the tunnel was built in the French colonial period and might reach Nui Nung site of the Botanical Garden near Ba Dinh Square.

The finding will help archaeologists verify the tunnel and then work out measures to restore and preserve it. Chu Van An (formerly named Buoi) High School has been recognised as a national historical relic.