Ischia ( , ]) is a volcanic island in the Tyrrhenian Sea. It lies at the northern end of the Gulf of Naples, about from the city of Naples. It is the largest of the Phlegrean Islands. Roughly trapezoidal in shape, it measures approximately east to west and north to south and has about of coastline and a surface area of . It is almost entirely mountainous; the highest peak is Mount Epomeo, at . The island is very densely populated, with 60,000 residents (more than 1,300 inhabitants per square km).