marine offensive reached its maximum intensity in 1916-1917,
during which period no less than 7000 mines were destroyed by the
British navy alone.[9] Of this number about 2000 were drifting when
discovered. There was, with one small exception, no portion of the coast
of the United Kingdom which was not mined at least once during those
eventful _two_ years, the unmined area being undoubtedly left clear to
facilitate a raid or invasion. About 200 minesweeping vessels were blown
up or seriously damaged, but the losses among the Mercantile Marine were
kept down to less than 300 ships out of the 5000 sailings which, on an
average, took place weekly.

The heavy losses inflicted on the enemy's submarine fleets in 1917
marked the turning of the tide, and from that date onwards there was a
steady but sure reduction in the number of mines laid.

During the first twelve months of the intensified submarine war the
Germans concentrated their mine-laying on the food routes from the
United States, the sea communications of the Grand Fleet off the east
coast of Scotland and the line of supply to France. Then, when they
commenced to realise the impossibility of starving the sea-girt island,
and the weight of the ever-increasing British armies began to tell in
the land war, the submarine policy changed to conform with the general
strategy of the High Command, and the troop convoy bases and routes were
the objects of special attack.

The arrival in Europe of the advance guard of the United States army
caused another change in the submarine strategy. From that time onwards
the Atlantic routes assumed a fresh importance and became the major zone
of operations.

In the first year of the war the U-C boats discharged their cargoes of
mines as soon as they could reach their respective areas of operation.
The mines were usually laid close together in one field, frequently
situated off some prominent headland, or at a point where trade routes
converged. Then the enemy learned to respect the British minesweeping
and patrol organisation, and endeavoured to lay their "sea-gulls' eggs"
in waters which had been recently swept, or where sweeping forces
appeared to be weak in numbers.

When this failed they played their last card, scattering the mines in
twos and threes over wide areas of sea. To meet this new mode of attack
large numbers of shallow-draught M.L.'s were employed to scout for the
mines at low water.

It was about this time that the