The island’s geography comprises relatively low-lying mountains surrounding a central plain, 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 medieval times. Today, the amount of land that is forested in Ireland is just one third of the European average of 35%. There are twenty-six extant mammal species native to Ireland.