In the heart of the San Juan Basin, in the arid north-western corner of New Mexico, stands one of the greatest ancient treasures in the US: the remarkably preserved remains of a vast building complex believed to have been constructed between 850 and 1250AD that may have housed as many as 5,000 people. The high desert area of Chaco Canyon sees cold winters, scorching-hot summers and only about 22cm of rainfall every year. Yet, it was home to a thriving, yet mysterious, civilisation – the Chacoans, ancestors of the Puebloans.

In 1907, this isolated 53sq-mile expanse of desert became a national historical park, containing 13 major ruins and more than 400 archaeological sites. Pueblo Bonito, the largest excavated site, covers about 2 acres and has roughly 800 rooms arranged in a D-shaped building. The site was designed with sophisticated irrigation systems, the doorways are perfectly aligned with each other to facilitate communication and researchers estimate it housed about 2,000 people.

Today, visitors can wander around the same maze of chambers and roads used by its residents roughly 1,000 years ago. Some of the structures used to create the roofs and different floors are long gone, but their remains clearly reveal how they were built. Approximately 200,000 wooden pillars were used in the construction and were most likely hand-carried from the Chuska Mountains and Mount Taylor, more than 112km away.

The Chacoan roads are another impressive feature of the complex. There are about 650km of them, some 9m wide, built mostly in straight lines that cut through the rough topography instead of going around it. Their positioning, starting at a central structure and running towards notable natural elements, such as lakes and mountains, suggests they represented symbolic connections between man and nature. Yet, it doesn’t mean they weren’t practical: studies have shown that walking on those roads is less tiresome than walking on the rough terrain next to them.

Over the years, archaeologists have come up with different theories for why Chaco was built. Although the place seems to have been a trade hub, the buildings suggest it may also have been an important ceremonial site and point to the Chacoans’ impressive astronomical knowledge. The walls are aligned with the axis of the sunrise on an equinox and the north doorway faces almost exactly true north. Contemporary New Mexico Pueblo tribes, such as the Zuni, still consider Chaco Canyon a sacred site and they return there for ceremonies.

Since 1987, the Chaco Culture National Historical Park, along with other smaller Chaco sites, is a Unesco World Heritage site . For the past few years, one of the biggest threats to the site has been the oil and gas drilling and mining around Chaco Canyon. In 2019, the Chaco Cultural Heritage Area Protection Act was introduced to the US Senate – if it passes, it will ban those activities within a 16km radius of the site.

(Video by Smoke & Apple; text by Luana Harumi)

This video is part of BBC Reel’s Remote Places playlist .

Join more than three million BBC Travel fans by liking us on Facebook , or follow us on Twitter and Instagram .

If you liked this story, sign up for the weekly bbc.com features newsletter called "The Essential List". A handpicked selection of stories from BBC Future, Culture, Worklife and Travel, delivered to your inbox every Friday.