Ischia is famous for its healing thermal waters, but the powers of its geothermal energy are less known – and they're hiding an ancient culinary tradition beneath the surface.

I'm wandering through the village of Sant'Angelo on the Italian island of Ischia, searching for the footpath that will take me to the volcanic fumaroles on the isle's southern shore – and towards one of Italy's most unique meals.

A shopkeeper points me to a stone staircase snaking up from an alleyway that climbs over the cliffs and mountains. The steep trail takes me past prickly pears and stucco villas, and as I trudge up to the clifftop, I admire the ink blue waters of the Gulf of Naples. On my descent to the fumarole-dotted beach, I approach the crescent-shaped Maronti Bay; umbrellas staked into ash grey sand.

I'm soon ushered to a corner table on the terrace of the seaside restaurant Chalet Ferdinando a Mare. Basil and tomatoes perfume the air as bathers frolic in the sea. I order lunch and a glass of wine.

Below the terrace is a patch of sand ringed by a fence. Danger signs glare in Italian and English: "Prohibited from entering to the unauthorised. Sand boiling 100C" (212F).

These are the fumaroles of Ischia, where sands are heated to a boil by underground volcanic vapours.

It's where my lunch is cooking right now.

Cooking meals in a fumarole is no culinary gimmick; it's Ischian tradition.

"My father always cooked under the sand," says Fernanda Iacono, the chalet's owner. "It's something we do in this part of the island. We cook chicken, potatoes, octopus, vegetables, fish, mussels… anything that cooks at low temperature."

Plan your trip:

Get there: Take the hydrofoil or ferry from Naples to Ischia Porto, then the CD, CS, or 1 buses to the Sant'Angelo stop (45 minutes). It is only possible to reach the fumaroles by foot or by sea. Walk or hail a pedicab down to the village (€8). Take a private boat or catch the water taxi at the docks (five minutes); the path leading to Maronti Bay is found next to Ragno boutique (20-25 minutes). Approach from the east on foot via Maronti Beach.

Do: Browse Sant'Angelo's posh boutiques, then soak at the Antiche Terme di Cavascura; a thermal spa carved into pumice rock. Ristorante Emanuela is open from April to November; Chalet Ferdinando a mare is open from June to the end of September. Reservations encouraged.

Don't: Attempt cooking in the sand without the chalet's approval (or a local's assistance).

Stay: The Miramare Sea Resort & Spa offers mesmerising sea views and full spa services.

Ischia teems with fumaroles, but they're hottest in Maronti Bay. Generations of Ischians have come here to cook, wrapping their food and burying it in the sand, where the steam acts as a sous vide. "We'd have parties on the beach where we cooked under the sand. With music, too," reminisces Iacono, who runs the chalet with her children, Giorgio and Desideria Migliaccio, and her son-in-law, Angelo Russo.

Islanders cook year round, but the tradition hits its zenith each summer.

"All Ischians do it at least once a year," says Mariangela Mattera, an Experience Expert at Ischia-based tour operator FORADAY. "In summer, especially at........

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