Camino Real de Tierra Adentro
Camino Real de Tierra Adentro is the historic silver mining route established by Spanish settlers in the 16th century. It begins in downtown Mexico City and runs through the several states in Mexico for 1,400 kilometers (870 miles) before crossing what is now the United States border at Juarez/El Paso and extending 650 km (404 miles) north to Santa Fe where it ends at San Juan Pueblo. Camino Real de Tierra Adentro is a 
UNESCO World Heritage Site.
The entire designated area of the route spans 2,600 km. The Mexican portion of the site is administered for the federal government by the INAH (Instituto Nacional de Antropologia y Historia), this portion includes 5 previously declared World Heritage Sites and adds an additional 55 as part of the Camino Real. The U.S. portion of the route is administered for the federal government by the Department of the Interior, NPS (National Park Service) and Bureau of Land Management (BLM); the U.S. portion does not inscribe specific sites, but includes historic areas of Santa Fe, including other designated trails.
Go
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Understand
editEl Camino Real de Tierra Adentro was a critical trade route that was actively used for 300 years, from the mid-16th to the 19th centuries. Missionaries followed the road north, building churches, convents, monasteries, and chapels along the way. Mule trains follwed the road as well. Cargoes of mercury imported from Europe would go north, and cargoes of silver from the mines of Zacatecas, Guanajuato, and San Luis Potosí, would go south for eventual shipment back to Spain. The UNESCO citation also acknowledges that the route fostered the "creation of social, cultural, and religious links in particular between Spanish and Amerindian cultures."
Get in
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Mexico
editThe route officially starts in downtown Mexico City. You could visit parts of the Camino Real de Tierra Real anywhere along the route.
The most scenic locations are in the Bajio region of Mexico where silver mines can be seen and where the wealth produced in the mines is ostentatiously displayed in the hundreds of colonial era churches, convents, temples, monasteries, and cathedrals. There are also some good history museums and a couple of old mines you can tour (like Mina Eden in Zacatecas). All of the towns in the Bajio have upscale posadas and good restaurants. Several towns are UNESCO World Heritage Sites, several others are designated as Pueblos Mágicos for their charm and touristic value.
You can easily use buses between cities from Mexico City north.
United States
editIn the U.S. section of the site, there are several historic sites in Albuquerque worth exploring, and Santa Fe is a charming town with cozy B&B inns. The site ends in San Juan Pueblo, New Mexico (also known as Ohkay Owingeh). It's an interesting place to see a bit of New Mexico pueblo culture and there's a historic church there. If you want to drive the route, you could follow US85 from El Paso, through Las Cruces and Albuquerque to Santa Fe. It's the original route used by the Franciscan padres, though they'd never recognize it given that most of it is now part of I-25.
Stay safe
editThe route is long and passes through some areas that have had chronic problems with drug trafficking and gang violence, particularly in cities like Ciudad Juarez near the U.S./Mexico border. Be aware of current issues and take appropriate safety precautions. It may be prudent to avoid areas of the Camino where security is an issue.