The first mining offensive against the Giand Fleet occurred in August 1915, when 38o mines were laid in the entrance to the Moray Firth, distributed over lines which totalled nearly 7o miles. A large portion of the fleet was at Invergordon at the time. The results were negligible, the losses being confined to minesweepers and one destroyer. A channel was found to be clear on the northern side of the Firth, which was at once used as an exit for the fleet. On the southern side, however, a channel was cleared, but an area of mines was purposely left to form a defensive bar rier and so limit the water requiring patrol and minesweeping. On January 1, 1916 the area west of Scapa Flow was mined by the "Mowe" on her passage into the Atlantic. In this field 252 mines were laid, endangering an area of 4o sq.m., which resulted in the loss of H.M.S. "King Edward VII." before the presence of mines was realised. Once located, this area was treated in the same way as the Moray Firth.
On May 29, 1916, "U.75" laid her 18 mines close to Marwick Head, off the northwest coast of Orkney. The operation was part of the pre-Jutland submarine actions of the enemy. It is proba ble that these mines were intended to be laid off the western entrance of Scapa Flow to hinder the exit of the fleet and that a mistake was made in the position. The mines were set for a depth of seven metres. The tide was appreciable causing sufficient "dip" to insure that the mines at this depth should be innocuous even to a heavy-draught ship, except at low water, and with consider able motion on that ship. The period of slack water was extremely limited. The "Hampshire," proceeding in a sudden gale, and hugging the shore to obtain less sea, struck one of the mines at low slack water and sank. By this curious conjunction of all these factors, the career of Lord Kitchener was brought to a tragic close. Except for a determined offensive by the Flanders sub marines against the Channel Ports, the mine was not extensively used in the latter part of 1916. It would appear that the Ger mans were then husbanding their resources for unrestricted sub marine and mine warfare.
Synchronous mining of adjacent British ports was resorted to by the Germans, which raised many sudden problems in traffic control. The convoy system had now been introduced, and the
sweeping of convoys into certain ports often became essential. Particular difficulties arose in regard to Liverpool owing to the shallowness of the channel and narrow entrance ; mines swept up, and sunk without exploding, causing serious danger to heavy draught ships by their proximity to the ship when resting on the bottom. The south coast of Ireland was seriously and continu ously mined, and even the bays on the west coast of Ireland did not escape.
Early in 1918, an appreciation of failure led the Germans to concentrate their efforts in minelaying in two directions only. The first was a grandiose scheme, which, commencing in Janu ary, was only concluded in late September. It consisted of batches of 36 mines, laid at regular intervals of Iom. apart on a semi circle 45m. from the Bell Rock, the result being a complete ring round the entrance to the Firth of Forth. This operation fulfilled a two-fold purpose. It menaced the exit and entry of Norwegian convoys, which were then working from Methil in the Firth of Forth; it also menaced every possible course taken by the Grand Fleet when leaving or entering its base at Rosyth. The scheme, however, was barren of results to the Germans, although method ically carried out in every detail. It was appreciated and coun tered after the third batch of these mines was discovered, the re sult being that each of the successive groups of mines was located and cleared immediately; and this was done without the Germans realising that any of these groups had been removed. The other concentration took the form of a field of 400 mines, also laid gradually by submarines. It was directed against the Dutch con voys, and was laid close to the Dutch coast to cover the ap proaches to the Maas and Ijmuiden. This position was such that any attempts at clearance exposed the minesweepers to a flank attack from enemy vessels. There were some losses by mine, par ticularly to destroyers escorting the convoys; but the convoys sailed as before, and the losses, on the whole, were very trivial. The total number of German mines laid in all parts of the world was 43,636.