Charting the northern seas The ancient world had only the vaguest of notions of the northern seas of Europe. The Atlantic Ocean and North Sea lay on the outer margins of the known world for the Greeks and Romans, and there were few seafarers from the Mediterranean world who ventured into these stormy waters. The Greek traveller Pytheas of Marseilles, driven into the North Sea by storms some time around 325 BC, described the tides he saw, probably off the coast of Friesland, as the rising and falling of the lungs of the sea. Julius Caesar’s first invasion of Britain in 55 BC almost came to grief when high tides – of which the Romans had little previous experience – waterlogged several warships drawn up on the beach and caused severe damage to the transport ships lying at anchor. Roman accounts of northern European waters invariably dwell upon their stormy and unpredictable character, and the second-century historian Appian even went so far as to claim that the Romans avoided the Atlantic or Germanic oceans, unless they had to go to or from Britain.