Arizona Mexico Border
The Arizona-Mexico border fence near Naco, Arizona, March 29, 2013. REUTERS/Samantha Sais

Illegal immigration across the U.S.-Mexico border is at a 40-year low, despite the raging national debate over the recent wave of Central American child migrants. U.S. authorities apprehended 420,789 undocumented immigrants in fiscal year 2013, the lowest number since 1974 and one-fourth of the highest figure recorded, 1.692 million in 1986.

But the border is still treacherous for those trying to pass through: U.S. Border Patrol agents have uncovered 97 bodies of migrants in the Arizona desert over the past 10 months.

That number doesn’t include statistics from the Texas-Mexico border, where the Border Patrol tallied 238 deaths last year out of a total of 445 along the Southwest border region. The U.S. Customs and Border Protection Agency has not yet released figures for migrant deaths along the Southwest border for 2014 so far.

“Most of the deaths occur in remote, hard-to-reach areas,” Tucson Border Patrol spokesman Andres Adame told news agency EFE. “If people fall and hurt themselves, they are left to die. Immigrants like to come over the mountains to avoid detection by the Border Patrol, but that's the most dangerous way to go because it's hard to rescue them.”

The 97 deaths in Arizona since October are actually a considerably lower figure than the roughly 200 bodies found in the Arizona desert in fiscal year 2013. But the majority of the corpses so far this year have been found during the summer, when migrants are most vulnerable to scorching desert heat during the long trek north.

The harsh desert region stretching across Arizona’s south represents some of the deadliest terrain for migrants coming into the United States from Mexico. A 2013 study by the Binational Migration Institute of the University of Arizona found that around 46 percent of migrant deaths in Arizona from 1990 to 2012 were due to exposure. The identities of most of the bodies found remain mysteries, with no information on names or countries of origin.

But in recent years, there has also been a surge of migrant deaths in Brooks County, Texas, as the numbers of Central American migrants coming into the Rio Grande Valley has continued to swell. In June, anthropologists discovered a mass grave in the Brooks County containing more than 160 unidentified migrants.

The death toll along the Southwest border has been on the rise since the 1990s, when the U.S. began to crack down on border enforcement. Under Operation Gatekeeper, a border security plan implemented under President Bill Clinton in 1994, the federal government ramped up Border Patrol staffing, fences and security equipment along the San Diego border area to deter migrants from crossing. But immigration activists say the move effectively pushed migration eastward into the more perilous mountain regions of eastern California and the Arizona desert, contributing to the heightened death toll over the last two decades.