League cliffs are among the highest sea cliffs in Europe, while Malin Head is the most northerly point on the island of Ireland. Two permanently inhabited islands, Arranmore and Tory Island, lie off the coast, along with a large number of islands with only transient inhabitants. The 129 km long River Erne, Ireland's ninth-longest river, enters Donegal Bay near the town of Ballyshannon. The River Erne, along with other Donegal waterways, has been dammed to produce hydroelectric power. A canal linking the Erne to the River Shannon was constructed between 1846 and 1860, creating Ireland's longest navigable waterway. The project was plagued with setbacks and closed in 1870, just ten years after its completion. A joint effort between the Irish and Northern Irish governments restored the canal in the late 20th century, and the Shannon–Erne Waterway reopened in 1994. Historically, the eastern boundary of the kingdom of Tír Chonaill was demarcated by the River Foyle. However, in the 17th century, an area of land west of the Foyle was given over to the newly established city of Derry; although the Foyle still demarcates a large section of Donegal's eastern border. To the south, the Drowes River forms a 9 km (6 miles) long natural boundary with County Leitrim. Donegal has a population density of 34.2 people per square kilometre, the lowest in Ulster and the fifth lowest in Ireland. The county has an expansive network of wildlife and conservation zones, which includes 46 European Union designated Special Areas of Conservation (SACs) and 26 Special Protection Areas (SPAs), as well as 14 Natural Heritage Areas (NHAs), 74 proposed Natural Heritage Areas, 145 Irish Geological Heritage Areas (IGHs) and Ireland's second largest national park, Glenveagh, which extends to over in northwestern Donegal. Owing to its scenic landscape and "world-class wilderness", Donegal was named National Geographic's "Coolest place on the Planet" in 2017. The forested area in the county extends to , the 4th highest total forest cover in Ireland. Historic deforestation left Donegal devoid of forest cover by 1900. Much of the county's forests are commercial timber plantations which were initially established in the 1930s as a way to create rural employment in areas with poor agricultural land and high rates of emigration. The county contains extensive tracts of blanket bog which are concentrated in western and upland regions. Blanket bog covers an area of , or roughly 30 percent of the