It's hot, smelly and you're bound to get dirty. Yet people pay a lot of money to get into this spa. For your next holiday, why not go to Hell?

Next time someone tells you to go to hell, rejoice. It can be heaven.

Forget the pain and agony. Picture a blissful spa, where you'll be pampered and regenerate both body and soul.

Italy boasts the world's most "hellish" natural thermal baths. They're dotted around the country, and for centuries, they've been likened to hell. Some were even banned, with the all-powerful Vatican wary of their powers. But today the sites celebrate their links to the devil's lair, with thousands visiting every year. Here are some of the hottest natural spas, as compiled by news.com.au.

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NERO'S STOVES, NAPLES

Prepare to boil like an egg inside Roman Emperor Nero's so-called "Stoves" near Naples. The Romans believed these caves to be the nostrils of Hades.

If you dare to go inside, the 50C temperature of the underground vapours oozing into these tiny, dark stone grottoes from the bellows of the Underworld will make you sweat away toxins, stress and, of course, sins.

They're natural saunas: It's like sitting inside a microwave covered in mineral-rich droplets that get inside your lungs.

From these inferno chambers, a labyrinth of tunnels connect to the open-air "Purgatory Lake", a green-water pool surrounded by crumbly Roman ruins and a lush park. It's a bit less boiling than the underground "stoves", but still pretty hot.

It's claimed floating in the thermal water will combat cellulite and make your skin smooth, to prepare you for the next stop — heaven.

There are two paradises to choose from — the underground bubbly pool where the water is cooler, or the soothing "Relaxation Cave" where you can enjoy a quick nap.

While these underground caves would indeed be hell for people who suffer from claustrophobia, this particular spa has nothing on the true Italian hell: Sicily's Vulcano island baths.

VULCANO ISLAND THERMAL BATHS, SICILY

Since mythological times, locals have believed this site to be the 'Mouth of Hell' where a cruel fire god lived. For centuries, the spa was banned by the Vatican — it deemed Christians shouldn't go near such a place of death.

At the entrance there's the meditation area, "Limbo Zone" — perhaps where you can ponder if you really want to venture any further. It precedes the "Inferno Pond", a hot mud bath of brown, slimy, disquietingly still pond with occasional geyser bubbles.

Once you enter, you are trapped in hell.

The pond floor is slippery with mud slime, which you must spread all over your body and face in order to fully atone for your misdeeds. Just be careful not to get it in your eyes and hair, as only lemon juice will remove it.

Floating like a jellyfish, you may get a little scorched on the holes where hot bubbles come out. But, we're assured, aching bones will be relieved and acne will disappear.

Old roman ruins of Thermal Spa at Villa Romana del Casale in Piazza Armerina, Sicily, Italy
Old roman ruins of Thermal Spa at Villa Romana del Casale in Piazza Armerina, Sicily, Italy

When the mud dries on your body, it's time to move to the "Purgatory Cavities", stove chambers within the yellowish-red volcanic rocks surrounding the pond. In the past, the nasty scent of sulphur contributed in keeping people away. Today the stench is a like a drug.

People sit for hours crouched inside the chambers, inhaling the fumes until they feel dizzy. Clouds of smoke can blur your vision. Warning signs tell people to stay no longer than 15 minutes.

When your time's up, dive into the "Paradise Inlet". The refreshing sea will bring you back to life, while the underwater fumarole spots are free jacuzzis.

But even these can be dangerous. Avoid floating above one for too long, as the vapours escaping can still knock you out and the hot bubbles can be worse than jellyfish stings.

GROTTA GIUSTI, TUSCANY

In Tuscany's Monsummano thermal park, you can swim inside the clear waters of the subterranean warm "Limbo Lake", practising a special aqua gym with a personal trainer who'll stretch your limbs until they're numb. It's dubbed "Spa Floating", and it's so relaxing you'll forget what your sins are.

Prepare to scuba dive, too, zigzagging through the underwater canyon, using yoga breathing techniques with a regulator.

The lake is the antechamber to the "Shades Realm", a huge, 200m-deep spa cave divided into three separate steam bath chambers — Paradiso, Purgatorio, Inferno. The deeper you go through a maze of pointed, jutting rocks, the hotter it gets.

Once you reach the steaming Inferno Room, collapse on the comfy wooden lounge chairs and indulge in your state of sublime damnation as vapours gently caress your body.

During the 50-minute stay in the netherworld consider your past, present — and hell, why not — future sins.