A concrete dug-out in a remote corner acts as the airport’s blast valve.

Any object suspected to contain explosives is kept in the dug-out — cooling pit in security parlance — for 24 hours.

“International security norms state that a suspicious object has to be kept in the dug-out — 4ft in length, width and breadth — for 24 hours. If there is any explosive, it will burst within that period,” said a security officer at the airport.

The bag which was suspected to contain explosives on Wednesday was kept in the pit, 2km north of the terminal buildings, throughout the night before being handed over to the owner.

Earlier this week, four containers on a Moscow-Singapore flight of Singapore Airlines, which was grounded at Calcutta following a bomb alert call, were kept in the pit for 24 hours.

“The pit is one of the most vital devices for the airport’s security,” said a senior security official at Calcutta airport.

A narrow, uneven road leads to the pit, which is cordoned off with barbed wire. The walls and the floor are of concrete while the top has an iron cover. “The pit is surrounded by sand bags as sand is one of the best shock absorbers. The sand will absorb the shock if there is an explosion and no splinter will come out of the pit,” said the official.

If any baggage is suspected to contain explosives during the X-ray check, it is immediately isolated. The area is cordoned off by CISF personnel and bomb detection and disposal squad officials are called.

The squad runs a check with vapour devices. Sniffer dogs, too, are brought in. Once the initial check is over, the baggage is covered with a “bomb blanket” made of fibre and carried to the pit on a special trolley.