It took officers a total of 50 hours to remove the two bombs located 27 metres below ground. Up to 6,300 people were evacuated from the surrounding areas and businesses had to be suspended during the operation.

Earlier this month, police were prompted to provide a second training session for about 80 MTR staff working on the site, with an emphasis on knowledge of bigger bombs. MTR Corporation also hired a company to conduct field inspections since the bombs were discovered, before construction work could resume.

Tony Chow Shek-kin, a senior bomb disposal officer overseeing both operations, said the deeper workers dug into the ground, the more likely frontline staff might encounter bombs as big as the two recently excavated.

Chow also made clear that the bureau, with only 11 full-time members, did not have enough manpower or professional equipment to conduct a comprehensive inspection of the MTR construction site, let alone the whole city.

“It’s not a difficult job,” said Daniel Hui, a barrister and former Royal Air Force pilot. “It’s not like finding a needle in a haystack. It’s more like finding the Michelin man in your classroom.”

Hui, who is also an amateur researcher of explosives and a student of the city’s wartime history, said the military details – the number of aircraft used, types and quantities of bombs dropped, targets of assaults and resulting damages – could be found in various archives and documents available locally and abroad.

One of the major sources Hui started with was the 10-volume British Army Aid Group series compiled by Elizabeth Ride, daughter of the group’s founder Lindsay Ride.

The paramilitary group was an officially classified unit that assisted prisoners of war to escape from Japanese concentration camps and collected military intelligence in southern China and Hong Kong.

The series contained field intelligence reports on the bombing of Hong Kong.

One of those, dated January 21, 1945, described a heavy raid on Wan Chai by 20 aircraft, “resulting in about 500 buildings damaged and around 1,700 Japanese naval personnel and 1,500 Chinese killed”. The apparent scene of the incident was an area at the back of the construction site where the two 450kg bombs were found.

Major evacuation continues after second bomb found at Hong Kong site where 450kg wartime explosive unearthed at weekend

“Can’t we find out who came, what units were deployed, and how many thousand-pound bombs were dropped? It’s not mission impossible but mission possible.” Hui said. “As a Hong Kong resident, are you satisfied that out there [in Wan Chai] they are going to teach workers to run around with a metal detector and look for bombs, and that’s it?”

In the 2000 book Memories of the Jing Bao and Beyond, former commercial pilot and amateur historian Ian Quinn estimated there were 1,000 unexploded bombs across Hong Kong. He also said the targets for the American air force were “Kowloon docks, Kai Tak and Taikoo docks”.

Philip Cracknell, a retired banker who is now a Hong Kong second world war historian, agreed that extensive surveys were needed at excavation sites as unexploded bombs “can blow up when hit by mechanical diggers”.

These areas, according to Cracknell, included the whole of the north shore of Hong Kong Island spanning Shau Kei Wan and Kennedy Town, Mount Davis, Aberdeen and Wan Chai Gap.

“I am sure there are still many more unexploded bombs in the urban areas and likewise on the hillsides ... I suspect unexploded ordnance would run into the many hundreds, and about 1,000 would be a reasonable estimate,” Cracknell said.

He added he worried that “it’s just a matter of time before someone gets killed or badly injured in Hong Kong from a [wartime] unexploded ordnance”.

Dave Macri, a visiting military history scholar at the University of Hong Kong, said it would be “very difficult” to find the remaining bombs scattered around the city.

“Action reports are not always exact ... finding the [bombs] would likely require expensive technology and specialists in explosive ordnance disposal,” Macri said.

One example the associate professor of history raised was the difficulty in pinpointing locations of bombs dropped amid the chaos of dive-bombing attacks, especially when there was fire from anti-aircraft guns on land.

But Hui argued that aircraft loaded with 1,000-pound bombs were usually “not manoeuvrable” and flew in a straight line at high altitudes, with radars or reconnaissance planes documenting the drops because the bombs were expensive.

Meet the heroes behind the 50-hour wartime bomb disposal operations in Hong Kong

Hui said considering the unknown risk to public safety, the city’s security authorities should look for historical archives, such as those held by the US, Britain and even Taiwan, to detect the unexploded bombs with higher accuracy and efficiency.

The Security Bureau, which does not have information on the number and locations of such bombs, told the Post that it was the responsibility of police when it came to issues over explosives.

Members of the public who suspect they may have found an unexploded ordnance are urged by authorities to leave the scene and call 999 immediately, and not to move the object by themselves.