The northern half of the world's second largest ocean is rich in islands, varied coastline and tributary seas. On the west are the Caribbean, the Gulf of Mexico, the Gulf of St. Lawrence, Hudson Bay and Baffin Bay. To the east are the Mediterranean, the Black Sea, the North Sea and the Baltic Sea. As well as purely oceanic islands there are those which rise from the Mid-Atlantic Ridge, including the Azores. Other islands such as The Canaries and Madeira rise from Africa's continental margins.