The tides of the bay of Saint-Malo are among the highest in Europe, especially during the twenty-two annual tides with a coefficient greater than 100. They are caused by an exceptional concentration of water in the heart of a triangular bay located between Brittany and Cotentin. During very high tide (coefficient 115), the sea can rise to 43.4 feet (13.25 meters) and drop to 0.65 feet (0.2 meter), i.e. a difference of more than 42.5 feet (13 meters) which corresponds to more than twice the ordinary tidal range in the Atlantic zone. The sea level change occurs in six hours ; the movement of the huge mass of water then generates very powerful and extremely dangerous currents.