[HW3]

I found this with a quick Google search....

From 'Engage the Enemy More Closely' - Barnrtt

From the first week of the war, however, Germany embarked on a campaign of offensive minelaying - ambushes beneath the sea Britain's own ports, coasts and naval bases. Most of these were o orthodox contact mines, of which the German Navy had over 20,000 in stock. One such minelaying operation, off the Tyne, by five destroyers covered by the cruisers Leipzig, Nurnberg and Köln, on 12-13 DeXcember gave the British submarine service the opportunity for biggest success so far in the war. The Salmon (Lieutenant-Commander E. O. B. Bickford), which had already sunk the U-36 with torpedoes, attacked the German squadron on its homeward voyage in the Heligoland Bight at dawn on the 14th, damaging Nurnberg and Leipzig so severely that they were out of action until May and December. 1940 respectively - a serious reduction in Germany's slender strength in cruisers during what was to be a decisive campaigning year at sea. Nonetheless, the German minelaying campaign in British waters, quite apart from causing vast disruption to coastal traffic, was to lead to the loss of 79 merchant ships, totalling 262,697 tons, in the first four months of the war.

As early as 16 September the damaging of the SS City of Paris by an underwater explosion confirmed Admiralty fears that the enemy was also using magnetic mines - that is, mines laid on the seabed and exploded by the influence of a ship's magnetic field on the mine -electro-magnetic detonating mechanism. In the shallow waters of the Continental Shelf round the British Isles, such seabed explosion could seriously damage or sink a ship.