In the interfluve of the Aravan and Ak-Buura rivers lies the unusual land of Chil-Ustun, where karst remnants, arranged in three rows, strikingly contrast with the green backdrop of the floodplains of the foothill valleys, which flow into the oasis of the Fergana Valley beyond the western border. From a geological, or more precisely, geomorphological perspective, the stone islands are remnants of a once large mountain massif. They are composed of sedimentary rocks from the Paleozoic era, which are about 350 million years old. The highest point is the summit of Mount Chil-Ustun, which rises 425 meters above the valley. The absolute height of the summit is 1460 meters. On the southern slopes, which are deeply dissected by narrow gorges, karst caves have been discovered along the layering cracks.
The most beautiful cave "Chil-Ustun" extends 380 meters and consists of three spacious halls connected by narrow corridors. The most impressive is the third hall, which is over 100 meters long, more than 20 meters high, and reaches widths of up to 50 meters in certain sections. The hall is supported by stunning columns that sparkle from the interplay of moisture and light. The walls are adorned with crystals and stone lace.
The crystals are not silent: they hold the enchanting music of past centuries, which begins to sound at the slightest touch.
20 km south of Chil-Ustun rises Mount Tuya-Muyun, which is cut by the Aravan River into a deep canyon. On the left side of the canyon are: the deepest cave in the republic - Fersman Abyss, with a depth of 240 meters, the famous Great Barite Cave, about 300 meters long, and the cave "Azhida-ar-Unkur" - home to a huge colony of bats.
On the right side are the caves: "Surprise" with a length of 450 meters and "Victory" extending 1200 meters. All the caves of the Tuya-Muyun massif are characterized by rich mineralization, unique to this area. Crystals of various colors and shapes adorn the surfaces of most underground halls and passages.
