One of Boston's more unusual public monuments is the unexploded (but presumably defused) mine [2] the Navy had put in the North Sea to blow up U-boats during World War I, now permanently on display next to the Soldiers and Sailors Monument at the top of Boston Common.
Links:
[1] http://www.universalhub.com/users/adamg
[2] http://www.celebrateboston.com/sites/boston-common-mine-sweepers-memorial.htm
[3] http://www.universalhub.com/beacon-hill
[4] http://www.universalhub.com/photos
[5] http://www.universalhub.com/taxonomy/term/427