SCIENCE said:

so

They all get found, eventually. Some just get found ‘the hard way’.

The Soviet battleship ‘Novorossiysk’ (formerly the Italian ‘Giulio Cesare’) was destroyed by a mine in Sevastopol harbour in 1955, probably a magnetic mine which had been missed in mine-clearing after WW2, and had lain dormant for 10 years until ‘Novorossiysk’ came close enough to set it off.

Searches after ‘Novorossiysk’ sank located nearly another three dozen similarly ‘overlooked’ mines in the harbour.