Prisoners participating in the Wild Horse Inmate Program train mustangs that will eventually be adopted by the U.S. Border Patrol, providing the agency with inexpensive but agile horses. The inmates on their turn will be equipped with skills and insights they hope to one day carry with them from Florence State Prison.

The horses are trained in Florence, Arizona and applied in border cities like Jacumba and also San Diego in California. They are critical for patrolling the rugged and remote stretches of the Mexican border to detect illegal crossings by migrants and drug trafficking. Just 654 miles (1053 kilometers) of fence exist between the United States and Mexico, accounting for about a third of the border.

The rest is defined by mountains, rivers, private ranches and wild country - terrain more suited for horses, which all agents had back when Border Patrol was founded in 1924. Here, Border Patrol agents head out on patrol along the fence near Jacumba, California.

On horseback, the agents can navigate desolate stretches of land that vehicles cannot. The mustangs are sure-footed on steep terrain, crossing creekbeds without hesitation and stepping spryly over rattlesnakes. Some 55,000 mustangs roam the Western United States. Here, wild horses are herded into corrals in Milford, Utah.

At the prison in Florence, a cactus-dotted town about 140 miles north (225 km) of the Mexican border, most inmates don't have experience with horses. Over the course of four to six months, the men train their horses to tolerate bridles and saddles, respond to commands to trot and canter and perform footwork that will come in handy on the uneven desert terrain along the border.

The task of the Florence inmates who train the horses is, at times, thick with irony: Some are Mexican nationals, apprehended on the border for drug-related offenses. The inmates, though, say they don't mind that the horses help law enforcement. They are simply happy the animals no longer face thirst and starvation in the drought-stricken West.

The San Diego border patrol unit has 28 horses, of which many were adopted from the Florence prison. These adoptions are key to the government's effort to stem the growing population of mustangs. A federal law tasked the Bureau of Land Management with managing wild horse and burro populations, both to protect the animals and to ensure that vegetation was not overgrazed and water sources depleted.

Florence began its horse training program in 2012, and while it is too early to assess the long-term effects on participating inmates, of the 50 or so who have gone through it and have been released, none has returned to prison.

"It really feels like the Wild West out where we patrol for sure," says Bobby Stine, supervisory agent of the San Diego Sector Horse Patrol Unit. "There's just not a lot of law enforcement presence, except for us."

The border wall with Mexico was the signature promise of now President Donald Trump's election campaign, standing out from the storm of controversial and often contradictory campaign statements. Once elected, Trump quickly set the wheels in motion by ordering the construction to start. Experiences from around the world, however, show that massive barriers do not come easy - or cheap.

The so-called "Tortilla Wall" already spans some 700 miles (1,126 kilometers) of the US-Mexico border. Experts believe that the US would need to pay between $15 billion and $25 billion in order to fully wall-off the entire southern frontier. US President Trump has said it would cost less and that Mexico would foot the bill.

Israel started building its own controversial barrier in 2002, with construction continuing to this day. The structure is expected to stretch at least 650 kilometers (403 miles) across the Holy Land, most of it consisting of an electric wire fence. Observers believe its cost has already topped $2.6 billion (2.4 billion euros), with maintenance costs reaching $260 million per year.

The Berlin Wall spanned 155 kilometers (91 miles) before its demolition in 1991. Unlike the many other walls across the world, the barrier was built by East Germany to keep the would-be emigrants inside the country. It cost about $25 million to build in 1961, equivalent to $200 million (almost 186.5 million euros) in present-day money.

Capitalist South Korea and its Communist northern neighbor are divided by barbed wire and watchtowers, as well as around 1 million landmines. Following the 1953 truce, both Pyongyang and Seoul agreed to pull their troops 2 kilometers (1.24 miles) away inland, creating a demilitarized zone along the border that stretches 248 kilometers (154 miles).

A total of 48 "peace lines" separate Catholic and Protestant neighborhoods in Belfast, a legacy of centuries-long religious war. The barriers including high brick walls, concrete structures, barb wire and metal bars. These barricades include gates to allow for circulation of people and traffic, but the gates close after nightfall.