National Geographic : 1917 Jan
HARBOR DEPTHS AT LOW TIDE s 25 feet and over S15 to 25 feet 3 Under 15 feet SHETLAND ISLANDS ORKNEY ISLANDS ^Kirkwall B3e~f sta3s I ubhul S Cork 0 50> 100 zo Nautical Miles SKETCH MAP INDICATING THE MULTITUDE OF BRITISH HARBORS The United Kingdom and Ireland contain 119 seaports, of which 80, even at low tide, are open to vessels drawing 14 feet of water. At average tide they will admit vessels requiring much greater depths. The seas surrounding the islands are very shallow, making it easy to anchor mines to destroy shipping and also to moor nets to trap submarines. If the waters of Dover Strait were to subside Ioo feet, an isthmus would connect England and Holland. If the waters subsided 300 feet, Ireland and the whole of the British Islands would once more be connected to Continental Europe.