Bras d'Or Lake, 1099 km2, is an arm of the Atlantic Ocean occupying the centre of Cape Breton Island that nearly divides the island in two. On the north it is linked to the ocean by a narrow channel down the west side of Boularderie Island. On the south it is divided from the ocean by a low strip of land that is breached by a ship canal at St. Peter's. This inland sea is about 90-95 km long and is circled by high hills.

Mi'kmaq were the earliest inhabitants and in 1650 a French trading post was built at St Peter's. Late in the 18th century, Loyalists settled along its shores, followed by Scottish immigrants. The lake has long been known for good fishing and today is popular for water recreation of all kinds. Baddeck, on the north shore, has been a shipbuilding centre. The name is a corruption of "labrador," a term applied by Portuguese explorers to a large part of Nova Scotia.