What are the oldest sea mines still causing problems at sea?
Short answerAn unconfirmed Crimean War mine in the Black Sea is the oldest report mentioned, with later wars also leaving persistent hazards.
NotesA reported Crimean War mine in the Black Sea is offered as a possible oldest surviving hazard, with its identification explicitly unconfirmed. American Civil War examples have also been reported. Mines from the Russo-Japanese War and both world wars are mentioned as lingering dangers. Laying hundreds or thousands makes complete accounting difficult: some drift away or otherwise escape the expected fate. A mine that fails to detonate can remain a hazard long after the operation that placed it.