The southeastern part of Cape Breton Island is home to the Sydney Coal Field, an extensive underground coal seam extending at an angle from the shore beneath the seafloor of the Cabot Strait. This large deposit of high-sulphur coal was first extracted by French soldiers from Fortress of Louisbourg in 1720 at nearby Port Morien. A major coal industry developed during the 19th century, becoming the largest energy project in British North America at its height of production. The largest integrated steel mill in the British Commonwealth was constructed on Sydney Harbor in 1901.