Bomb disposal experts from the Royal Navy transported a large, unexploded bomb down the River Thames on Monday night, then detonated it at sea.

Airport security the world over has changed dramatically in the years since the tragic events of September 11th, 2001, particularly in nations with close ties to the United States like the UK. On Sunday, however, as the London City Airport was shut down and evacuated following the discovery of a 500-kilogram (more than 1,100 pound) bomb found near the runway, the threat wasn’t terrorism, a lone wolf or a disgruntled employee… it was our own history.

“Following the discovery of a World War Two ordnance in King George V Dock as part of planned development works, a 214m exclusion zone has been implemented as a precaution by the Met Police. As a result, London City Airport is currently closed.” Officials from the London City Airport posted on Twitter.

The bomb, measuring nearly five feet long, was found buried in the dense silt abutting the runway, where it had laid dormant since it was placed there during the air raid campaigns of World War II. It is likely that the bomb may have been dropped by German aircraft attacking what was once a London industrial center that stood where the airport is now located.