Ancient walls being eaten by salt-loving bacteria. The microbes extract nutrients from the clay itself, creating decay patterns that look like biological circuitry.
The living desert walls
The ancient city of Khiva, a UNESCO World Heritage site, is a city of earthen architecture rising from the Kyzylkum desert. Its famous crenellated walls, mosques, and madrasas are built from sun-dried mud-bricks—a simple mixture of local clay, sand, water, and chopped straw. For centuries, these materials have endured the extreme desert climate, which can reach over 45°C in the summer. A more persistent threat is degrading these structures from within: a community of microorganisms.
The primary driver of this decay is salt. Khiva suffers from a high water table, a consequence of its location near the Amu Darya river and extensive regional irrigation. Groundwater, rich in dissolved mineral salts, is drawn up into the porous mud-brick walls through capillary action. As the water evaporates from the surface, it leaves behind salt crystals in a process called efflorescence. The physical pressure from these growing crystals creates micro-fractures in the bricks, weakening the entire structure. This high-salinity, arid environment creates a perfect niche for a specialized class of microbes known as halophiles, or "salt-lovers."
Biological circuitry
The patterns of decay on Khiva's walls are not random; they are the visible result of a hidden ecosystem. Microbial communities, composed of salt-tolerant bacteria and archaea, colonize the salt-crusted surfaces. These microorganisms are chemoorganotrophs, meaning they derive energy by oxidizing organic compounds. The main source of food is the straw that was originally mixed into the bricks as a binding agent.
As the microbes consume the organic temper, they break down the brick's internal structure. Their metabolic activity, combined with the physical action of the salts, creates complex patterns of decay that trace the paths of moisture through the walls, resembling biological circuits. Clay minerals within the bricks play a dual role in this process. The large surface area of clay particles provides a habitat for microbes to colonize and form protective biofilms. These biofilms can trap moisture and nutrients, concentrating the biological activity in specific zones. Scientists studying similar biodeterioration on other ancient structures have identified various bacterial and fungal genera capable of penetrating deep into brick substrates. Understanding this interaction between the geology, hydrology, and microbiology of Khiva's walls is the first step for developing conservation methods that can halt the city's slow dissolution.
💡Fun Facts
The mud-bricks of Khiva are unfired, meaning they are simply dried in the sun rather than baked in a kiln.
Some of the wooden columns supporting the city's Juma Mosque date back to the 10th century.
The high groundwater in the region is so close to the surface that, for centuries, residents have traditionally built above-ground tombs instead of digging graves.
The inner walled city, known as the Ichan-Kala, was the first site in Uzbekistan to be declared a UNESCO World Heritage Site in 1990.
The streets of the Ichan-Kala are open 24/7, but museums and monuments are generally open from 8:00 AM to 6:00 PM daily.
Admission
Entry to the walled city itself is free, but a ticket is required to access the museums, minarets, and walls. A standard ticket costs around 100,000-150,000 Uzbekistani som and is valid for two days.
Accessibility
The terrain inside the Ichan-Kala consists mainly of cobblestone streets and packed earth, which can be uneven. Many historic buildings have narrow staircases and high thresholds.