It was fairly easy to deal with this method as a dangerous area was proclaimed round the spot where a mine was discovered, and experience soon showed the necessary extent of area to proclaim.

Later the submarines laid mines in groups of about six.

This necessitated the proclamation of more than one area, and was naturally a more difficult problem.

At a further stage the submarines scattered their mines in even smaller numbers, and the task of ensuring a safe channel was still further increased.

The most difficult artifice to deal with, however, was the introduction by the Germans of a delay action device in their mines, which caused them to remain at the bottom for varying periods after being laid.