Most historic sea mines are encountered on beaches. They do not come into contact with areas of dense population and therefore are not as problematic as unexploded bombs buried under cities and towns. For example, when a live German sea mine was found at a Sussex beach last month, disruption was minimal.
However, if mines wash up close to port infrastructure the ramifications can be significant. In 2015 two mines encountered close to Calais caused the closure of the port and cancellation of multiple ferry crossings.
A Royal Navy bomb disposal team was sent to check this mine even though they had been told by the Coast Guard that it was a training weapon. You can’t be too careful when dealing with suspected UXO of this size. Had it been a live air-delivered mine, there would have been a very large associated hazard.
British WWII air-delivered mines measured approximately 9 feet by 17 inches and weighed in at around 680kg. Half of this weight was the amatol or minol HE charge; a very large quantity of explosive.