Many years ago, I sent one of my technicians out with our "industrial" type metal detector used for detecting burried manhole covers, had a penetration depth of around 2m..
He digs up in someone's back garden what looks like part of small aeroplane tail and brings it back to the office thinking he's found the remains of a plane crash. After examining said bit of metal I deduced that it was either from a plane for extremely small people, about the size of a large action man, someone pissing about or part of a second world war bomb but as it was East Dulwich and not even the Germans bothered bombing that shite hole it was just left in the corner. A few weeks later we, our term contractor actually did find a bomb near the Thames, big b*stard, bomb disposal people out, area cordoned off, etc, etc. Anyway I told one of the bomb disposal guys about our "find" and he came to have a look. Turns out it is a known uxb (unexploded bomb) from the second world war, they even knew the size! So some poor guy and his family are sitting on top of a 250kg unexploded second world war bomb!! A week or two following this he sent me a list of all the known unexploded bombs in London, a question was asked in the houses of parliament apparantely so the info is freely available if you do a google search, theres a 500kg bomb under one of the major south London hospitals