In the 2011 Norway terror attacks, Oslo was hit by a bomb blast that ripped through the Government quarter, damaging several buildings including the building that houses the Office of the Prime Minister. Eight people died in the bomb attack. Oslo occupies an arc of land at the northernmost end of the Oslofjord. The fjord, which is nearly bisected by the Nesodden peninsula opposite Oslo, lies to the south; in all other directions Oslo is surrounded by green hills and mountains. There are 40 islands within the city limits, the largest being Malmøya (), and scores more around the Oslofjord. Oslo has 343 lakes, the largest being Maridalsvannet (). This is also a main source of drinking water for large parts of Oslo. Although Eastern Norway has a number of rivers, none of these flow into the ocean at Oslo. Instead Oslo has two smaller rivers: Akerselva (draining Maridalsvannet, which flows into the fjord in Bjørvika), and Alna. The waterfalls in Akerselva gave power to some of the first modern industry of Norway in the 1840s. Later in the century, the river became the symbol of the stable and consistent economic and social divide of the city into an East End and a West End; the labourers' neighbourhoods lie on both sides of the river, and the divide in reality follows Uelands street a bit further west. River Alna flows through Groruddalen, Oslo's major suburb and industrial area. The highest point is Kirkeberget, at . Although the city's population is small compared to most European capitals, it occupies an unusually large land area, of which two-thirds are protected areas of forests, hills and lakes. Its boundaries encompass many parks and open areas, giving it an airy and green appearance. Oslo has a humid continental climate (Köppen climate classification "Dfb") with warm summers and cold winters. Due to oceanic influences, winters are less cold than more continental areas at same latitude. With isotherm, it is also reasonable for Oslo to be classified as a borderline oceanic climate. Oslo has a significant amount of rainfall during the year. This is true even for the driest month. Because of the city's northern latitude, daylight varies greatly, from more than 18 hours in midsummer, when it never gets completely dark at night (no darker than nautical twilight), to around 6 hours in midwinter.