Located along the Douro river estuary in Northern Portugal, Porto is one of the oldest European centres, and its historical core was proclaimed a World Heritage Site by UNESCO in 1996. The western part of its urban area extends to the coastline of the Atlantic Ocean. Its settlement dates back many centuries, when it was an outpost of the Roman Empire. Its combined Celtic-Latin name, Portus Cale,[12] has been referred to as the origin of the name "Portugal", based on transliteration and oral evolution from Latin. In Portuguese, the name of the city is spelled with a definite article ("o Porto"; English: the port). Consequently, its English name evolved from a misinterpretation of the oral pronunciation and referred to as Oporto in modern literature and by many speakers.