The area of Budapest lies in Central Hungary, surrounded by settlements of the agglomeration in Pest county. The capital extends in the north-south, east-west direction respectively. The Danube enters the city from the north; later it encircles two islands, Óbuda Island and Margaret Island. The third island Csepel Island is the largest of the Budapest Danube islands, however only its northernmost tip is within city limits. The river that separates the two parts of the city is wide at its narrowest point in Budapest. Pest lies on the flat terrain of the Great Plain while Buda is rather hilly. The wide Danube was always fordable at this point because of a small number of islands in the middle of the river. The city has marked topographical contrasts: Buda is built on the higher river terraces and hills of the western side, while the considerably larger Pest spreads out on a flat and featureless sand plain on the river's opposite bank. Pest's terrain rises with a slight eastward gradient, so the easternmost parts of the city lie at the same altitude as Buda's smallest hills, notably Gellért Hill and Castle Hill. The Buda hills consist mainly of limestone and dolomite, the water created speleothems, the most famous ones being the Pálvölgyi cave (total length ) and the Szemlőhegyi cave (total length ). The hills were formed in the Triassic Period. The highest point of the hills and of Budapest is János Hill, at above sea level. The lowest point is the line of the Danube which is above sea level. Budapest is also rich in green areas. Of the occupied by the city, is green area, park and forest. The forests of Buda hills are environmentally protected. The city's importance in terms of traffic is very central, because many major European roads and European railway lines lead to Budapest. The Danube was and is still an important water-way and this region in the centre of the Carpathian Basin lies at the cross-roads of trade routes. Budapest is one of only three capital cities in the world which has thermal springs (the other being Reykjavík in Iceland and Sofia in Bulgaria). Some 125 springs produce of thermal water a day, with temperatures ranging up to 58 Celsius. Some of these waters have medicinal effects due to their medically valuable mineral contents. Budapest has a humid continental climate (Köppen climate classification that borders on "Cfa"-"Cfb"-"Dfa"-"Dfb"), with relatively cold winters and warm summers according to the 1971–2000 climatological norm when the 0 °C isotherm is used.