All flights today at London City Airport were cancelled after a bomb was found in the River Thames. The bomb is actually a German 500kg fused device that's been sitting in the Thames since the Germans dropped it during World War II. The unexploded ordnance was discovered during work on a dock near the airport. The Royal Navy is working to remove the bomb. From NPR:
The discovery of World War II era bombs in London is not particularly rare, as NPR's Ari Shapiro has reported. "During the Blitz, German planes dropped nearly 30,000 bombs on London in just three months," he notes.
In 2015, a German bomb of about the same size was discovered in an east London neighborhood, prompting an evacuation.
At that time, Matt Brosnan, a historian at the Imperial War Museum, told the BBC that we don't know exactly how many of the bombs dropped could still be hidden.
"Clearly not all of those would have exploded, because of defects or other reasons, and they could have buried themselves tens of feet below the surface so we simply don't know where they are," he told the broadcaster.
"World War Two ordnance found in the Thames" (Metropolitan Police)