Chile's Tarapacá region sits near the borders with Bolivia and Peru. The region's hyperarid Atacama Desert has been compared to Mars.

The desert soil contains a mix of chemicals carried by groundwater, ocean spray, or fog. A lack of rainfall helped preserve beds of sodium nitrate, or saltpeter.

In the early 19th century, Europeans on the hunt for saltpeter to use in gunpowder turned their attention to the desert.

Companies were mining saltpeter in the region by the early 1800s. When Charles Darwin visited during his voyage on the HMS Beagle in 1835, he dismissed the Chilean version of saltpeter.

"This saltpetre does not properly deserve to be so called; for it consists of nitrate of soda, and not of potash, and is therefore of much less value," he wrote.

The desert's sodium nitrate was more prone to dampness and burned at a higher temperature than potassium nitrate, another type of saltpeter commonly used in gunpowder, John Darlington wrote in "Amongst the Ruins: Why Civilizations Collapse and Communities Disappear."

Extracting nitrate and valuable byproducts like iodine quickly became industrialized. It required a slew of machinery, including hoppers, leaching tanks, and troughs.

As populations grew and scientific development continued, researchers turned to sodium nitrate not as a source of gunpowder but as a useful fertilizer.

By 1870, the Tarapacá region was producing 500,000 tons of saltpeter, the largest source of the substances in the world at the time, the BBC reported.

The Peruvian Nitrate Company founded La Palma in 1862, and nearby Santa Laura followed 10 years later. La Palma was later renamed Humberstone, after a British chemical engineer, James Humberstone, who moved to the area in the 1870s.

Many European investors set up operations in the Tarapacá region, trading a share of their profits to acquire the land.

One of the driest places on earth, some parts of the Atacama Desert receive only 0.2 inches of rainfall a year, according to the National Oceanic and Atmospheric Administration. While the average temperature is in the high 60s Fahrenheit, it can drop to close to freezing at night in the winter.

Humberstone is arranged in a 10-by-6-block grid. Many buildings were made of Douglas fir with zinc roofs. Verandas and covered walkways provided relief from the sun.

A general store and hotel were built at the center of Humberstone on the plaza. A tennis court, theater, swimming pool, chapel, hospital, and school all made up the complex.

Some of the materials for the buildings were shipped in, but workers also used remnants of the saltpeter process for "Pampa cement."

The workers came from Chile, Peru, and Bolivia. Barracks were constructed of dorm-like rooms for single workers. There were also small houses for families.

Managers lived in larger, nicer homes, while owners usually had homes in coastal cities instead of near the mines.

Workers were lured to Humberstone and other mining towns by companies promising to pay their travel costs. "But what they found was really harsh conditions, very rough work, very dangerous work, and very poor pay," Ángela Vergara, a professor of history at California State University, Los Angeles, told Business Insider.

Administrators would also physically punish the workers.

One reporter who visited in the 1880s described the work as incessant, comparing it to the dirty, dangerous work of coal mines. He called houses "squalid-looking," per the BBC.

Rather than being paid in money, workers were paid in tokens that were only accepted in the town's general store.

"That was to trap people," Vergara said. "They could not move because they were dependent on the token."

Saltpeter workers in Humberstone and other towns became an important part of the country's labor movement.

By the end of the 19th century, nitrate railways connected the region's mines to port cities so saltpeter could be shipped all over the world. At the same time, the miners were dependent on the railroads, too.

"All these nitrate camps and towns, they were located in very isolated parts of the Atacama," Vergara said. They had to bring in food and supplies to survive, she said.

Port cities shipped saltpeter to Europe and other parts of the world and imported goods, including textiles and coal.

Between 1880 and 1930, "Chile literally lived off one product: saltpetre," historian Julio Pinto told BBC News in 2015.

It brought in about half of the country's fiscal revenue, he said.

Chile went to war with Bolivia and Peru over nitrate taxes in 1879, eventually annexing nitrate-laden territories from both countries, including Tarapacá.

Bolivia was cut off from the coast, becoming the land-locked country it still is today.

Germany relied upon Chile's saltpeter for fertilizer until the British blockade during the war. Instead, German scientists found ways to synthesize nitrate from ammonia, bypassing the need for saltpeter.

The loss of the industry combined with the Great Depression had a severe effect on Chile's economy. While it had once produced 80% of the world's nitrate, by 1950 it was only responsible for 15%.

A private company bought the operations and then sold off pieces of them in 1961. Workers left to find other jobs, leaving ghost towns behind.

The saltpeter works became Historic Monuments in 1970, saving them from demolition. In the 1990s, former workers and their families formed the Saltpeter Museum Corporation and won the rights to the sites during a public auction in 2002.

The Saltpeter Museum Corporation and former residents gathered 20,000 signatures to have UNESCO recognize the historical significance of the sites.

The pampinos, the area's inhabitants, had a unique culture influenced by the mix of people from all over the world. The towns and the industry represented specialized knowledge that impacted the landscape and deeply affected the country's economy.

Together, the two towns showcase different aspects of the region's former saltpeter industry. Santa Laura's equipment and manufacturing structures are better preserved than Humberstone's, which still has many residential buildings and other remnants of social and cultural life.

Between the 1960s and early 2000s, there was little maintenance on the towns' buildings. Looters took reusable materials, and the elements damaged the fragile structures, which weren't built for long-term use.

Salty fog had corroded the metal, and wind and earthquakes were threatening the wooden and stucco structures.

The Ministry of Public Works, the Saltpeter Museum Corporation, and the National Council of Monuments started working on securing and conserving the sites in 2005. In 2019, UNESCO removed them from its List of World Heritage in Danger due to their efforts. Some buildings are still fragile, though.

Many buildings, including the school and general store, remain. Visitors can wander around Humberstone and Santa Laura, which are only about half a mile from each other.

Mannequins depict what it would have been like to live and work in the towns.

Sources for this story include "Amongst the Ruins: Why Civilizations Collapse and Communities Disappear," UNESCO World Heritage Convention, "Tangible and Intangible Heritage in the Age of Globalisation," Applied Geochemistry, Chemical and Engineering News, Astrobiology, The Hispanic American Historical Review, and BBC News.