If you’re in or around Rome’s Trastevere neighbourhood at midday, you may be startled by the sound of a very loud bang.

It’s not the start of World War III, but the Cannone del Giancolo (Janiculum Cannon) on top of the Janiculum Hill, from which a blank is fired every day at 12pm by soldiers in military uniform.

The tradition of firing a cannon at midday first began in 1847, when it was introduced by Pope Pius IX in order to synchronise the Vatican’s church bells.

To let the officer in charge know when the cannon should be fired, a painted black wicker ball was hauled to the top of a wooden rod on the Church of Sant’Ignazio, and dropped at exactly midday.

The officer would watch through binoculars, and give the signal to fire as soon as he saw the ball fall (these days, the signal is given remotely). 

READ ALSO: Did you know…? Rome is home to a pyramid

The ritual actually started at Castel Sant’Angelo, where it continued until 1903. The cannon was moved to Monte Mario for a year, and finally to the Janiculum Hill, where it remains to this day.

The practice was temporarily put on hold for obvious reasons during World War II, but brought back in 1959 due to popular demand.

If you’ve ever been to the  Janiculum terrace that overlooks Rome, you’ll notice that there’s a giant monument to Giuseppe Garibaldi, the republican general who led the campaign to unify Italy, as well as the busts of numerous other generals.

So does the cannon have anything to do with these war memorials? Well, sort of…

In 1849, a major battle was fought on the Janiculum Hill between Garibaldi’s republicans, who were fighting to end papal rule over the city and create a united Kingdom of Italy, and Napoleon’s French army, who backed the pope.

The French actually won that battle, forcing Garibaldi and his troops to retreat to the republic of San Marino.

But the resistance remained very much alive, and in 1870 the republican army retook Rome, finally bringing an end to the Papal States and establishing Rome as the capital of a unified Italy.

At first, the cannon fired from Gianicolo was one of the original cannons used by the republicans to fire on the gate of Porta Pia in a key episode that led to the surrender of the pope’s army in 1870.

In 1991, however, it was replaced with a WWII howitzer that’s loaded with a kilo of gunpowder every day – enough to generate plenty of smoke and noise, even if (fortunately) there’s no actual cannonball involved.