Naval minefields could not be laid in dense fields, thanks to the risk of 'countermining'. Countermining was where the detonation of one mine set off other mines in the same field, as the shock of the detonation could either set off the trigger mechanisms, or cause sympathetic detonation of the explosives in other mines. This meant that a ship hitting a mine would set off the entire field, removing the threat to other ships. Countermining could also be induced deliberately, as a minesweeping measure. As a result, minefields were typically broadly spaced. One of the most deadly fields of WWI, the Ottoman minefield in Eren Keui Bay (which sank three pre-dreadnought battleships and damaged a battlecruiser on the 18th March 1915), was a line of 20 mines, stretching over about 2.5 km. As such, the average spacing between each mine was ~125 m.