(CNN) — Ever since the world's first oil well was dug here back in 1847, Azerbaijan has been making the most of its plentiful stocks of "black gold."
More than 170 years later, Azeri Light crude oil is still the country's most valuable resource.
But in the little town of Naftalan, 320 kilometers west of Baku in the foothills of the Lesser Caucasus Mountains, an unusual local oil -- also called Naftalan -- has been attracting the travel set for hundreds of years.
This crude oil is too thick for commercial use and it isn't much good for burning. But it's said to have health-giving properties and so people have been bathing in it since ancient times.
Recently, treatments involving being submerged in the dark gloop have had a resurgence.
'Thick blood of the Earth'
The nine-hotel Naftalan resort was founded in 1926, at the dawn of Azerbaijan's Soviet era when physical fitness was a popular passion.
These days, spring and fall are the peak seasons, when the hotels are fully booked with guests arriving from all over the world.
It's easy to see why -- roses are in bloom throughout the health resort, while butterflies fly through the open windows along with fresh mountain air.
Around 15,000 people a year come here to sit in baths of warm oil, soaking up what locals call "the thick blood of the Earth."
Naftalan oil is reputed to ease more than 70 skin, joint and bone diseases. It's used as a disinfectant, an anti-inflammatory and even as an anesthetic.
"It is proven to be an effective treatment of the diseases of musculoskeletal system, skin diseases, gynecological and urological diseases and especially good results reached in treatment of rheumatism," Azer Vagifov, the main medical practitioner of Qarabagh Spa-Hotel, tells CNN Travel.
Naftalan oil is unique to the region.
Locals have long known of the therapeutic benefits of Naftalan oil.
The earliest record is in the 12th century, when Azerbaijani poet Nizami Ganjavi wrote of it being exported to neighboring countries through traders' caravans.
Legend has it that its healing properties were first discovered when a caravan camel fell sick and was left behind.
The camel rolled into an oil pool and when the caravan returned weeks later the creature was discovered by its herder, fully recovered.
In the 13th century, Italian explorer Marco Polo wrote in "The Travels" of the oil used for "anointing the camels in maladies of the skin, and for other purposes; for which reason people come from a great distance for it."
The science bit
So what makes this oil so special?
Its effectiveness is mostly credited to its unusually high concentration of naphthalene (around 50%), a hydrocarbon that's an active ingredient in coal tar soaps -- often used to treat skin conditions such as psoriasis and eczema -- but also commonly used as a moth repellant.
At Naftalan Resort, treatments are preceded by a full medical checkup. The procedure isn't advisable for those with heart conditions or diseases of the nervous system, or undergoing cancer treatment.
After the initial blood test, ultrasound and inhalation tests are carried out. If no restrictions are detected, the patient is then directed to their first oil bath.
The oil treatment can be taken externally and internally, depending on the patient's condition and doctor's prescriptions.
The average age of patients is around 40 years old, but children as young as six are permitted to take the treatment.
While no link has been found between oil bath treatments and cancer, EU and US regulations have both deemed naphthalene a potential carcinogen for humans, as does the International Agency for Research on Cancer (IARC) of the World Health Organization (WHO).
Naftalan Resort says that the treatment is harmless and has been scientifically proven, citing that there are around 1,500 scientific research projects and 270 dissertations on the subject.
Hot and fragrant
If you've ever wondered what it's like to sit in a bathtub full of crude oil, the sensation's probably not too dissimilar to being in hot chocolate, given that the oil in the tub is a piping 39 C (102 F).
Warmed-up crude is undeniably fragrant, but while some complain about the smell, it's really not that overpowering.
One guest who traveled here from St Petersburg with her son told CNN Travel that she was willing to endure the sulfuric odor because "you come here with psoriasis but leave with beauty."
Each bath lasts 10 minutes, with courses of treatments running anywhere from three days to three weeks.
You may take your bath half full, or it could be filled up to your neck -- it all depends on the prescription of your physician.
If a full body bath isn't recommended, there's equipment for applying the treatment locally on arms and legs.
Trying to do anything yourself is strictly prohibited and is frankly impossible, due to the fact that moving around in a huge puddle of oil is naturally very slippy.
Guests relax after their treatment.
After the procedure is complete, an assistant will use a scraping brush to help remove the coating of oil and they'll then lead you to the shower area. The oil takes a fair amount of washing and scrubbing to remove, and the fragrance persists.
Whether it's a reaction to the 39 C heat or the oil itself, the immediate sensation after the procedure is fatigue and and patients are advised to rest for at least 40 minutes.
There's a lot more to the Naftalan Resort than oil baths.
Depending on the treatment plan, guests might supplement their visit with physiotherapy, hydro-massages, paraffin treatments and aromatherapy hydrobaths.
Then there's the cedar barrel sauna, said to intensify the already metabolism-boosting properties of an oil bath treatment.
There's also an on-site gym, yoga and Pilates studios, an outdoor pool, outdoor massage areas a vitamin bar and a tearoom.
The long-term result of the procedures is said to be visible within a month, but some patients report seeing results after just taking three baths.
One guest who had been suffering from back pain and joint pain in his legs said he was so motivated by his improved health conditions, he walked about a kilometer to the Naftalan river to fish with his friends.
A woman, Alena, said it was her fourth visit in two years and that the baths gave her a chance to live life fully, without pain.
And if you've been wondering what happens to the oil after someone's bathed in it -- it's recycled. It all goes back to a communal tank for future bathers.
"Black gold" is a precious commodity, after all.
Prices depend on your choice of resort, hotel and treatment. At Qarabagh Spa-Hotel, the minimum seven-night stay costs 189 AZN ($111) for one person or 229 AZN ($134) for two people. That includes meals, a medical check up, Naftalan baths, physiotherapy and use of the spa.