Naval mines were typically designed to disable themselves, or make themselves safe, after a particular time. The most common way to do this was to have a water-soluble plug in the casing. Over time, this dissolved, and eventually allow water to flood the mine and cause it to sink. However, these plugs were unreliable, and could also leave the mine still active on the bottom. During WWII, the British began to deploy mines containing a small explosive charge with a clockwork timer. This was a much more reliable way to ensure that the mines made themselves safe. Influence mines (those using magnetic, pressure or acoustic detonation) could also have so-called 'sterilisers' fitted. These were, fundamentally, devices that short-circuited the mine's battery, causing it to lose charge over time. Eventually, the battery would go flat, and the mine's firing mechanism would be rendered inert. Finally, merely just being sat in seawater would slowly cause a mine to corrode and decay. The typical active lifespan for a mine was about a year before it sank.

While these systems could disable mines, they were not fitted to every mine that was dropped or laid. They were also not fully reliable, especially the sterilisers fitted to influence mines. This meant that, at the end of WWII, there were large fields of mines that had to be swept. According to the Hague Convention of 1907, each belligerent was responsible for removing the mines that it had laid. They also had to share information on the mines they had laid in their opposition's waters. This was done after WWII, to some extent. The Royal Navy cleared the many mines it had laid along the east coast of the UK, while the USN cleared American mines in the Pacific. The southern part of the North Sea was cleared by the German Mine Sweeping Administration (GMSA) and successor organisations. The GMSA was a quasi-military force, formed by the Royal Navy from ex-Kriegsmarine sailors and using a combination of British and German equipment. It was completely under British control, with a British central staff giving orders to German subordinates. The GMSA was responsible for clearing mines along the German North Sea coast, as well as the coasts of the Netherlands, Denmark and Norway. While it did good work, the Soviets objected to it, seeing it as a precursor to a reformation of the Kriegsmarine. It was disbanded in 1948, and replaced with a fully civilian organisation, the Cuxhaven Mine Sweeping Group, which had similar duties. The work took a long time. The Thames Estuary was not declared mine-free until 1948, while the Mediterranean would not be cleared until 1950. Waters that were declared to be mine-free were not necessarily safe. In 1949, a ship would be sunk by a mine off Dunkirk. While the waters had been swept, declared to be clear and had been in use for several years, the mine had been missed; it had been covered by sand after laying, and then uncovered by a change in the currents. There are areas that are still at risk today. The Baltic, the scene of dense minelaying by both the German and Soviet navies, is the main focus of this. Estimates of the nimber of mines remaining in the Baltic range between 50,000-100,000, and there are frequent NATO exercises that aim to reduce that number further.