London City Airport grounded all flights on the night of February 11, after a World War II bomb was found in the neighboring River Thames, The Guardian reports.

The half-ton bomb was revealed Sunday morning by development work taking place at the King George V Dock. Following its discovery, police set up a 702-foot exclusion zone around the area, closing local roads and shutting down the London City Airport until further notice. According to the BBC, 261 trips were scheduled to fly in and out of London City Airport on Monday. Some flights are being rerouted to nearby airports, while others have been canceled altogether.

The airport will reopen as soon as the explosive device has been safely removed. For that to happen, the Met police must first wait for the river's tide to recede. Then, once the bomb is exposed, they can dislodge it from the riverbed and tow it to a controlled explosion site.

The docks of London’s East End were some of the most heavily bombed points in the city during World War II. Germany’s Blitz lasted 76 nights, and as the latest unexpected discovery shows, bombs that never detonated are still being cleaned up from parks and rivers more than 75 years later.

[h/t The Guardian]