Ireland is the third-largest island in Europe and the twentieth-largest island in the world. It lies to the north-west of continental Europe and is surrounded by hundreds of islands and islets. To the east of Ireland is the island of Great Britain, separated from it by the Irish Sea. The Republic of Ireland covers five-sixths of the island. Northern Ireland, a part of the United Kingdom, covers the remainder and is located in the northeast of the island. The population of Ireland is estimated to be 6.2 million. Slightly fewer than 4.5 million are estimated to live in the Republic of Ireland and slightly fewer than 1.8 million are estimated to live in Northern Ireland. This is a significant increase from a modern historical low of 4.2 million in the 1960s but still much lower than the peak population of over 8 million in the mid-19th century prior to the Great Famine. Relatively low-lying mountains surrounding a central plain epitomise Ireland's geography with several navigable rivers extending inland. The island has lush vegetation, a product of its mild but changeable oceanic climate, which avoids extremes in temperature. Thick woodlands covered the island until the 1600s. Today, it is the most deforested area in Europe. Twenty-six mammal species are native to Ireland, with some, such as the red fox, hedgehog and badger, being very common. Others, like the Irish hare, red deer and pine marten are less so. A Norman invasion in the Middle Ages gave way to English domination by the 1500s. In the 1700s, a system of Protestant English rule was designed to materially disadvantage the Roman Catholic majority and Protestant dissenters. In 1801, Ireland became a part of the United Kingdom. A war of independence in the early 20th century led to the partition of the island, creating the Irish Free State, which became increasingly sovereign over the following decades. Northern Ireland remained a part of the United Kingdom and saw much civil unrest from the late 1960s until the 1990s. This subsided following a political agreement in 1998. In 1973, both parts of Ireland joined the European Community. The Republic of Ireland experienced unprecedented economic growth from the mid-1990s until the 2008–2010 Irish financial crisis. Irish culture has had a significant influence on culture world-wide, particularly in the fields of literature and, to a lesser degree, science and learning. A strong indigenous culture exists, as expressed for example through native sports and the Irish language, alongside a common Western culture, such as contemporary music and drama, and sports such as rugby and golf.