<P> During the 1960s and 1970s, severe power shortages, strikes, and a violent Marxist--Maoist movement by groups known as the Naxalites damaged much of the city's infrastructure, resulting in economic stagnation . The Bangladesh Liberation War of 1971 led to a massive influx of thousands of refugees, many of them penniless, that strained Kolkata's infrastructure . During the mid-1980s, Mumbai (then called Bombay) overtook Kolkata as India's most populous city . In 1985, prime minister Rajiv Gandhi dubbed Kolkata a "dying city" in light of its socio - political woes . In the period 1977--2011, West Bengal was governed from Kolkata by the Left Front, which was dominated by the Communist Party of India (CPM). It was the world's longest - serving democratically elected communist government, during which Kolkata was a key base for Indian communism . In the West Bengal Legislative Assembly election, 2011, Left Front was defeated by the Trinamool Congress . The city's economic recovery gathered momentum after the 1990s, when India began to institute pro-market reforms . Since 2000, the information technology (IT) services sector has revitalised Kolkata's stagnant economy . The city is also experiencing marked growth in its manufacturing base . </P> <P> Spread roughly north--south along the east bank of the Hooghly River, Kolkata sits within the lower Ganges Delta of eastern India; the city's elevation is 1.5--9 m (5--30 ft). Much of the city was originally a wetland that was reclaimed over the decades to accommodate a burgeoning population . The remaining undeveloped areas, known as the East Kolkata Wetlands, were designated a "wetland of international importance" by the Ramsar Convention (1975). As with most of the Indo - Gangetic Plain, the soil and water are predominantly alluvial in origin . Kolkata is located over the "Bengal basin", a pericratonic tertiary basin . Bengal basin comprises three structural units: shelf or platform in the west; central hinge or shelf / slope break; and deep basinal part in the east and southeast . Kolkata is located atop the western part of the hinge zone which is about 25 km (16 mi) wide at a depth of about 45,000 m (148,000 ft) below the surface . The shelf and hinge zones have many faults, among them some are active . Total thickness of sediment below Kolkata is nearly 7,500 m (24,600 ft) above the crystalline basement; of these the top 350--450 m (1,150--1,480 ft) is Quaternary, followed by 4,500--5,500 m (14,760--18,040 ft) of Tertiary sediments, 500--700 m (1,640--2,300 ft) trap wash of Cretaceous trap and 600--800 m (1,970--2,620 ft) Permian - Carboniferous Gondwana rocks . The quaternary sediments consist of clay, silt, and several grades of sand and gravel . These sediments are sandwiched between two clay beds: the lower one at a depth of 250--650 m (820--2,130 ft); the upper one 10--40 m (30--130 ft) in thickness . According to the Bureau of Indian Standards, on a scale ranging from I to V in order of increasing susceptibility to earthquakes, the city lies inside seismic zone III . </P> <P> The Kolkata metropolitan area is spread over 1,886.67 km (728.45 sq mi) and comprises 3 municipal corporations (including Kolkata Municipal Corporation), 39 local municipalities and 24 panchayat samitis, as of 2011 . The urban agglomeration encompassed 72 cities and 527 towns and villages, as of 2006 . Suburban areas in the Kolkata metropolitan area incorporate parts of the following districts: North 24 Parganas, South 24 Parganas, Howrah, Hooghly, and Nadia . Kolkata, which is under the jurisdiction of the Kolkata Municipal Corporation (KMC), has an area of 185 km (71 sq mi). The east--west dimension of the city is comparatively narrow, stretching from the Hooghly River in the west to roughly the Eastern Metropolitan Bypass in the east--a span of 9--10 km (5.6--6.2 mi). The north--south distance is greater, and its axis is used to section the city into North, Central, and South Kolkata . East Kolkata is also a section . </P> <P> North Kolkata is the oldest part of the city . Characterised by 19th - century architecture, dilapidated buildings, overpopulated slums, crowded bazaars, and narrow alleyways, it includes areas such as Shyambazar, Hatibagan, Maniktala, Kankurgachi, Rajabazar, Shobhabazar, Shyampukur, Sonagachi, Kumortuli, Bagbazar, Jorasanko, Chitpur, Pathuriaghata, Cossipore, Sinthee, Belgachia, Jorabagan, and Dum Dum . The northern suburban areas like Baranagar, Noapara, Dunlop, Dakshineswar, Nagerbazar, Belghoria, Sodepur, Madhyamgram, Kestopur, Birati, Khardah up to Barrackpur are also within the city of Kolkata (as a metropolitan structure). </P>

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