The United Kingdom of Great Britain and Northern Ireland (commonly known as the United Kingdom, the UK, or Britain) is a collection of member states, to the north-west of mainland Europe. It comprises the island of Great Britain, the north-east part of the island of Ireland and many small islands. Northern Ireland is the only part of the UK with a land border, sharing it with the Republic of Ireland. Apart from this land border, the UK is surrounded by the Atlantic Ocean, the North Sea, the English Channel and the Irish Sea. The largest island, Great Britain, is linked to France by the Channel Tunnel.