Each year, thousands of illegal migrants try to enter the United States via a treacherous journey across the Arizona desert. Some succeed, while others are captured by US border patrols and are immediately deported - but not everyone is so fortunate. A growing number simply drop dead from exhaustion.
The Missing Migrant Project works on identifying the deceased, piecing together clues found in the personal effects collected alongside the decomposed bodies found in the desert.
In this programme, the BBC's Mexico correspondent Will Grant travels to Tucson, Arizona to meet project co-founder Robin Reineke to learn of the challenges facing her office in the small southwestern city of Tucson - which has the third-highest number of unidentified bodies in the United States, after New York and Los Angeles.
Migrant rights groups say the vast expansion of the US Border Patrol has exacerbated the problem because the heightened policing of the border along traditional urban crossing points has forced clandestine border crossers out into the wilds of the desert.
Such tough border protection is popular among many American voters, especially in conservative border states like Arizona and Texas - but some locals have shown sympathy, heading out into the desert to leave water, food and blankets in the hope of saving the lives of desperate migrants.
In Mexico, Crossing Continents also meets the relatives of those who have died in the desert, revealing their motivations to move north - motivations which they share with many men, women and children from across Latin America, who are still willing to risk their lives embarking on this increasingly dangerous and potentially deadly trip.
Reporter: Will Grant Producer: Richard Fenton-Smith.