Porto or Oporto is the second-largest city in Portugal, the capital of the Porto District, and one of the Iberian Peninsula's major urban areas. Located along the Douro River estuary in northern Portugal, Porto is one of the oldest European centres, and its core was proclaimed a World Heritage Site by UNESCO in 1996, as 'Historic Centre of Porto, Luiz I Bridge and Monastery of Serra do Pilar'. The historic area is also a National Monument of Portugal.