Ultimately closed in 1993, both the coking plant and the mine have been inscribed in the list of UNESCO World Heritage Sites since 2001. Notable accomplishments of the city in recent years include the title of European Capital of Culture on behalf of the whole Ruhr area in 2010 and the selection as the European Green Capital for 2017. Essen is located in the centre of the Ruhr area, one of the largest urban areas in Europe (see also: megalopolis), comprising eleven independent cities and four districts with some 5.3 million inhabitants. The city limits of Essen itself are long and border ten cities, five independent and five "kreisangehörig" (i.e., belonging to a district), with a total population of approximately 1.4 million. The city extends over from north to south and from west to east, mainly north of the River Ruhr. The Ruhr forms the "Lake Baldeney" reservoir in the boroughs of Fischlaken, Kupferdreh, Heisingen and Werden. The lake, a popular recreational area, dates from 1931 to 1933, when some thousands of unemployed coal miners dredged it with primitive tools. Generally, large areas south of the River Ruhr (including the suburbs of Schuir and Kettwig) are quite green and are often quoted as examples of rural structures in the otherwise relatively densely populated central Ruhr area. According to the Federal Statistical Office of Germany, Essen with 9.2% of its area covered by recreational green is the greenest city in North Rhine-Westphalia and the third-greenest city in Germany. The city has been shortlisted for the title of European Green Capital two consecutive times, for 2016 and 2017, winning for 2017. The city was singled out for its exemplary practices in protecting and enhancing nature and biodiversity and efforts to reduce water consumption. Essen participates in a variety of networks and initiatives to reduce greenhouse gas emissions and to improve the city's resilience in the face of climate change. The lowest point can be found in the northern borough of Karnap at , the highest point in the borough of Heidhausen at . The average elevation is . Essen comprises fifty boroughs which in turn are grouped into nine suburban districts (called "Stadtbezirke") often named after the most important boroughs. Each Stadtbezirk is assigned a Roman numeral and has a local body of nineteen members with limited authority. Most of the boroughs were originally independent municipalities but were gradually annexed from 1901 to 1975.