"Lie down! Lie down now!” the boatman shouted, and we pressed ourselves against the floor of the boat.

The cliff face loomed ahead of us, and I could see the tiny opening we were heading for. It looked impossible to get through.

At first, everything was black and I could hear the gentle lapping of waves echoing around the cavern. Then, as I raised myself above the sides of the boat, I saw the water. It was glowing a bright turquoise, the most intense blue I have ever seen – a mesmerising sight.

The Grotta Azzurra, as it is known in Italian, has been a wonder since Roman times, when Emperor Tiberius used it as his private swimming pool. A subterranean passage connected it to his villa on the Capri hillside, and anyone who was caught swimming in it was executed.

But its fame declined with the Roman empire. Local fishermen avoided it, believing it was inhabited by evil spirits, and it was largely forgotten, until, in 1826, it was rediscovered by two German swimmers. Sunlight refracted through the sea into the cavern gives the water its magical blue colour.

The easiest way to reach the Grotto Azzurra is by boat, and a boat trip is also the best way to take in the beautiful rugged coastline of Capri.

The island’s craggy limestone cliffs are covered in tiny yellow flowers. In places, the steep slopes have been cultivated with lemon trees and olive groves, and white buildings cling to the hillside.

At one end of the island are the distinctive Faraglioni rock stacks, the largest of which was used as a lighthouse by the ancient Greeks.

The coastline is steeped in myth and legend, and Capri competes with Sorrento, on the mainland, in its claim to be home of the sirens – the mythical creatures who lured sailors to their deaths with their singing.

The coastal town of Sorrento is built on steep cliffs and looks out across the Bay of Naples to Vesuvius. The centre of the town is a labyrinth of narrow streets with shops selling leather goods, pottery and coral jewellery, as well as sweets, liqueurs and soap made from the lemons the region produces.

No trip to the region would be complete without a visit to Pompeii. The city, preserved in up to 6m of volcanic ash after nearby Vesuvius erupted, is well-known, but nothing had prepared me for the sheer scale of the place.

It is thought to have been home to around 20,000 people in 79 AD, when Vesuvius obliterated it, and the site stretches over 66 hectares, around 45 of which have been excavated. Buildings range from public baths to temples and beautiful villas with mosaics on the floor, as well as huge public squares, theatres and amphitheatres.

Despite the grand public buildings, it was the smaller features of the city that I found most interesting. There were ruts in the paving stones where horses and carts had passed, and you can still see metal runners that enabled artisans to slide back the doors of their workshops.

Perhaps most fascinating of all are the ‘bodies’ of the inhabitants, frozen in the positions in which they died after being overcome by the poisonous gases that accompanied the eruption. I had always assumed the volcanic ash had simply mummified people where they fell, but the truth is even more amazing.

The city was largely forgotten in the years after the eruption, and was only rediscovered in 1738, with excavations beginning in 1764.

In 1860 an archaeologist, Giuseppe Fiorelli, noticed that he could identify cavities below the earth, often by tapping his stick on the ground. He then made tiny holes over them and pumped plaster in. Once the plaster had set, he painstakingly excavated it to reveal the shapes left by bodies that had long since rotted away.

The people of Pompeii had no sense of their impending doom. They thought Vesuvius was simply a mountain.

Classic Collection Holidays offers seven nights’ B&B at the Grand Hotel Excelsior Vittoria, Sorrento, during October with return flights ex-Manchester to Naples at £1,229. Call 0800 294 9324, or visit classiccollection.co.uk.