Pergamum or Pergamum (/ˈpɜːrɡəmən/ or /ˈpɜːrɡəmɒn/; Ancient Greek: Πέργαμον), also referred to by its modern Greek form Pergamos (Greek: Πέργαμος), was a rich and powerful ancient Greek city in Mysia. It is located 26 kilometres (16 mi) from the modern coastline of the Aegean Sea on a promontory on the north side of the river Caicus (modern-day Bakırçay) and northwest of the modern city of Bergama, Turkey. During the Hellenistic period, it became the capital of the Kingdom of Pergamum under the Attalid dynasty in 281–133 BC, who transformed it into one of the major cultural centres of the Greek world. Many remains of its impressive monuments can still be seen and especially the outstanding masterpiece of the Pergamum Altar. Pergamum was the northernmost of the seven churches of Asia cited in the New Testament Book of Revelation. The city is centered around a 335-metre-high (1,099 ft) mesa of andesite which formed its acropolis. This mesa falls away sharply on the north, west, and east sides, but three natural terraces on the south side provide a route up to the top. To the west of the acropolis, the Selinus River (modern Bergamaçay) flows through the city, while the Ketios river (modern Kestelçay) passes by to the east.