Literally

Then the third and final contribution from Sascha's Azerbaijani adventure. 

He now heads for the south of the country with a report on the thermal springs of Masalli, otherwise known as Istisu.

What to expect?
The wikivoyage  entry on the (nearby) city of Lankaran:

'Istisu Hot Springs.  
Some hot springs (istisu) near east of Lankaran of the same name as the sanatorium further away. Situated in a natural area with oak and iron-wood trees and a small stream'.

goazerbaijan:

'If you are interested in thermal waters, therapeutic lakes, springs and caves, you have to visit Azerbaijan. You can not only relax, but also to improve your health. One of the most famous thermal springs is Istisu health-resort in Lankaran region, in Masalli. Today in Istisu there are several health-resorts, which are aimed at both budget and vip-guests. The first thing you notice upon arrival at Istisu is the smell of sulfur and steam, swirling above the ground. Therapeutic sulfuric water exits the depths of the earth, and its temperature is above 60 degrees.  The composition of natural spring – hydrogen sulfide, sodium chlorine, calcium, magnesium carbonate, and each liter of water contains 30 milligrams of iodine.
...
Hot water from the thermal springs flows into small individual pools or baths, in which one can stay no longer than ten minutes. Meanwhile, the water temperature is not higher than 35-40 degrees'.

Azerbaijan.travel:

'Masalli is known for its mineral springs, the most famous of which is Istisu (literal meaning 'hot water'). This thermal spring appears in several places along the Vilashchay River bank on the Dombal hillside at 1,650 metres above sea level. The water comes out hot (about 69 degrees) from a depth of 90-250 m and is used to treat many diseases. Today, there's a sanitorium called Fatima Zahra, as well as guesthouses, teahouses and other tourist facilities'.

More details from cbt.az:

'Isti su – is one of the therapeutic water of Azerbaijan. It is situated at 13 km distance form Masalli city, on the bank of Vilash river. This water contains hydrogen sulphite, sodium chlorite, magnesium, hydrocarbonate and 30 milligrams of iodine in 1 litre. This water is extracted from 90-250 km depth of the earth at temperature more than 60 degrees. The following diseases can be treated with this water: rheumatism and its complications (arthritis, polyarthritis, joint pains, arthrosis); skin diseases (psariosis, dermatitis, neurodermatitis, fungus, eczema); diseases of kidney and urinary tract (pielitis, uretritis, pielonefritis, pielocystitis); radiculitis; prostate; hypertension; diseases of liver and bile duct; female diseases (metritis, endometritis, erosion of the neck of uterus, herbicide, colpitis); female and male childlessness; gastrointestinal diseases (gastritis, esofogitis, colitis, intestinal defecation); traumatic injuries'.

So quite a bit of info available.

Classic

How about the experience? Sascha:

"In the South of Azerbaijan close to Masalli are several hot springs. 

One of them is part of a sanatorium where I stayed for one night. 

The thermal water is captured and used for small private pools and bathtubs in small houses. In other houses mainly women are offering massage, the accommodation includes 3 meals but the bath and the massage are extra charged. It is advisable not to soak more than 15 minutes per day.

Do not expect the classic soviet style sanatoriums, it is more a couple of houses and huts close by the river on the rocks.

On the other side of the river is a building with other hot springs (see picture below). Here, the water is coming from the source with gas that burns. On the hill is a small field with burning gas that comes out of the ground.

 

The entrance fee is 5 Manat (€2,5).

A system of hot thermal water (45°C) and cold water permits to find the right temperature.

In Isti-Su you are not obliged to stay in the sanatorium (although it is a cheap option to stay), for 1 Manat you can park the car on the main place and walk down either to the bathing rooms or to the river. From there, head to the left and walk up the river. After 300m or 400m you will find natural hot springs that are coming out the rocks. 

Unfortunately, the rain was again too strong that it was impossible to soak inside. I understood that small pools in the river give the possibility for a nice soak".

 

What a bath looks like:

Witness
I've got a couple more experiences to share.
Herderlicemil notes (2014):

'Istisu is a hydrogen-sulfide treatment center. The local people call the water a “miracle”. Hot water of 69o C comes from under the ground here. This water is used to treat problems with joints and muscles, skin, gynecological and other diseases. The local sanatorium has mi­neral water baths as well. The water coming from rocks at Istisu is ice cold. Rich in hydrogen-sulfide, it smells like rotten eggs. It is poured into a glass and drunk in one gulp. There are “kidney water” and “stomach water” signs above the springs. Spring water can heal ulcer and kidney stones. The local treatment center recei­ved the status of a sanatorium in 2007. Due to its subtropical climate, the rate of precipitation in Masalli is higher than elsewhere in the region. Frequent rains make the Talysh forests even more ro­mantic. Istisu forests are 12 km from the center. Istisu is beautiful at any time of the year. In autumn, one can see all colors of na­ture here. There are not many hotels working in this season. To witness the entire beauty of Istisu, it is worth visiting in spring, autumn and winter. There are fewer people and the relatively cool weather makes the local baths very ple­asant. Whereas it is difficult to spend more than three to five minutes in the bathtub in the summer, one can swim for hours when the heat is not oppres­sive. The feeling after an Istisu bath is one of relaxation. If you have wounds or gashes on your body, you can see what wonders the water can work.
In the summer the forest looks different as scores of people come here for treat­ment in July and August.
Istisu is also famous for its waterfalls. They make the place even more gor­geous. Hiding deep inside forests, wa­terfalls make their surroundings a little cooler, which attracts people to picnics'.

An additional experience of the same (?), fizzoflife:

'On the way back, this time downhill to Lankaran, we came across our final treasure of the day, the natural sulphur hot springs at Haftoni settlement. The Istisu (hot water) signs took us up and along ever narrowing rocky dirt tracks for much longer than we expected, so that we were beginning to wonder whether we were hopelessly lost in the woods about to enter Iran, perhaps, when the ‘baths’ suddenly appeared, a collection of small and simple individual bath houses. We had not brought a towel with us but one was soon produced from a washing line. Our friend Karen took off into the woods to take photographs while the lovely husband and I entered our own little bathhouse. There was absolutely nothing fancy about it, I can assure you, and it smelled, well yes, slightly of rotten eggs, but it had a fairly large plunge pool with very hot water, hotter in temperature than a normal bath, say. Here we wallowed for three minutes at a time, then rested on a bench for a further three, three times successively. Sulphur baths are beneficial for all sorts of things, from skin complaints to damaged ligaments and arthritis, and I must admit, I felt a million dollars when I emerged from my bathing session. My asthmatic breathing was much improved and the lovely husband’s painful sprained ankle felt almost healed. This wonderfully therapeutic experience cost AZN3 per head, including the loan of the towel. As an added bonus, we both slept like logs that night'.