They're faily common but only the bigger or more disruptive ones make the news, there was one last year[1] that closed a few bridges [1] https://www.theguardia

They're faily common but only the bigger or more disruptive ones make the news, there was one last year[1] that closed a few bridges [1] https://www.theguardian.com/uk-news/2017/jan/19/wwii-bomb-discovery-closes-waterloo-and-westminster-bridges https://www.theguardian.com/uk-news/2017/jan/19/wwii-bomb-di...