It would have been tasteless to have called it a Blitz spirit but there was an air of stoicism over north-west London last week after a remnant of war prompted school closures and the evacuation of hundreds of homes – and a day of reflection for residents.

The unexploded bomb – the latest to emerge more than 75 years after ordnance rained down on the capital – lay on the site of a new block of flats. While army bomb disposal experts worked through Thursday night to make it safe, residents gathered at a church outside the exclusion zone. Ten schools were closed on Friday.

Robin Mills watched builders leaving the site in Brondesbury half an hour before the police came knocking. He spotted the device, a rusting barrel with a conical tip, and told the Kilburn Times that being evacuated felt “a bit strange”. He added: “I can imagine what people in the war times must have felt.” More than 20,000 bombs fell on London in the Blitz, which began in September 1940 and reshaped several other cities. Estimates vary, but about one in five bombs are thought not to have detonated. Safe unless disturbed violently, they also serve as time capsules, preserving fragments of the fear and disruption of war beneath our feet.

Just days earlier, police raced to Shoreditch in east London after reports of a device, which turned out to be a false alarm. Last week in Portsmouth, offshore dredging revealed a bomb, which was later detonated on the seabed. In January, two bridges were closed near the Houses of Parliament after the discovery of a suspected bomb in the Thames.

But most bombs or unexploded ordnance turn up on buildings sites. Figures are scant, but in 2009 the Construction Industry Research and Information Association estimated that, between 2006 and 2009, about 15,000 devices, including grenades, had been removed from construction sites. Five per cent were live, equivalent to roughly 250 devices a year.

It would make sense if discoveries are becoming more frequent; in many cases, original sites are being redeveloped for a second time after blocks thrown up in the postwar building boom reach the end of their useful life. Many of the worst hit areas are now sites of explosive redevelopment. And as buildings get taller, foundations go deeper.

In 2012, Dr Catherine Jones, a geographer now based at the University of Strasbourg in France, led a project to gather archive and geographic data to create Bomb Sight, an online map of strikes on London in an eight-month period. For the first time, this presented a zoomed-out view of the city drowned in a sea of red markers. “It’s a narrative that’s well known, that Europe was heavily bombed, but to see that kind of visualisation somehow connects you emotively,” Dr Jones says. “I got goosebumps the first time I saw it.”