Istisu is a sanatorium located in Masalli district, its full name is Istisu “Fatimeyi-Zahra” Healing Sanatorium.

Istisu “Fatimeyi-Zahra” Healing Sanatorium is located 13 kilometers away from the city of Masalli, at an altitude of 1650 meters above sea level , on the breast of Dambalov mountain, on the bank of Vilash river. It contains hydrogen-sulfide, sodium-chlorine calcium, magnesium-hydrocarbonate and 30 milligrams of iodine per liter. The solution comes out of the very deep layers of the earth when it is hotter than 60 degrees. This solution cures diseases without treatment. Local residents have used this water as a treatment since ancient times.