The Germans never issued such a warning unless the proclamation stating that half the Atlantic Ocean, most of the North Sea, and nine-tenths of the Mediterranean were dangerous could be considered as such.

It was also intended, as mines became available, to lay more deep minefields in positions near our own coast in which enemy submarines were known to work; these minefields would be safe for the passage of surface vessels, but our patrol craft would force the submarines to dive into them.

This system to a certain extent had already been in use during 1915 and 1916.
Schemes were also being devised by Admiral of the Fleet Sir Arthur Wilson, who devoted much of his time to mining devices, by which mines some distance below the surface would be exploded by an enemy submarine even if navigating on the surface.
Such was the policy.

Its execution was difficult.
The first difficulty lay in the fact that we did not possess a thoroughly satisfactory mine.

A percentage only of our mines exploded when hit by a submarine, and they failed sometimes to take up their intended depth when laid, betraying their presence by appearing on the surface.
Energetic measures were adopted to overcome this latter defect, but it took time and but few mines were available for laying in the early months of 1917.
The result of our minelaying efforts is shown in the following table: Mines laid Deep mines laid Year.