Ischia is a volcanic island in the Tyrrhenian Sea. It lies at the northern end of the Gulf of Naples, about 30 kilometres from the city of Naples. It is the largest of the Phlegrean Islands. Roughly trapezoidal in shape, it measures approximately 10 km east to west and 7 km north to south and has about 34 km of coastline and a surface area of 46.3 square kilometres (17.9 sq mi). It is almost entirely mountainous; the highest peak is Mount Epomeo, at 788 metres. The island is very densely populated, with 60,000 residents.