Biscay, Bay of, an arm of the Atlantic Ocean between southwestern France and northern Spain. It has a maximum width of about 350 miles (560 km) and reaches a depth of more than 16,700 feet (5,090 m) some 100 miles (160 km) north-northwest of Santander, Spain. The bay is subject to severe storms. Tides along the shores are among the world's highest, ranging up to 40 feet (12 m). Major cities on or near the bay include Bordeaux and Nantes in France and Bilbao in Spain.