Sunset in cape cod with duck in the water. Cape Cod (or simply the Cape) is an arm-shaped peninsula nearly coextensive with Barnstable County, Massachusetts and forming the easternmost portion of the state of Massachusetts, in the Northeastern United States. Although the Cape was originally connected to the mainland, the Cape Cod Canal, which opened in 1914, effectively transformed Cape Cod into a large island. Three bridges cross this canal from the mainland to the Cape. Cars can cross on the Sagamore Bridge and the Bourne Bridge; the other is a railroad bridge.