The geography of Ireland reflects its situation as an island in northwest Europe in the north Atlantic Ocean. The ocean is responsible for the rugged western coastline, along which are many islands, peninsulas and headlands. The main geographical feature of Ireland is low central plains surrounded by a ring of coastal mountains. There are a number of sizable lakes along Ireland's rivers, with Lough Neagh the largest in either Britain or Ireland. The island is bisected by the River Shannon, at 259 km (161 mi) with a 113 km (70 mi) estuary the longest river in either Britain or Ireland, which flows south from northwest County Cavan to meet the Atlantic just south of Limerick. The island of Ireland consists of the Republic of Ireland and Northern Ireland. Located west of the island of Britain, it is approximately 53° north of the equator and 8° west of the Greenwich meridian. It has a total area of 84,116 km² (32,477 mi²). Ireland is separated from Britain by the Irish Sea and from mainland Europe by the Celtic Sea.