The border between the United States and Mexico stretches 3,145 kilometers (1,954 miles), from the Gulf of Mexico to the Pacific Ocean. In some places, the border is only marked by a sign or a fence. In other places, the border is reinforced with barbed wire or tall steel barriers.

One of the most strongly fortified areas of the border separates the urban areas of San Diego, in the U.S. state of California, and Tijuana, in the Mexican state of Baja California. Here, the border is marked with double and even triple fencing. The first fence, pictured above, is about 3 meters (10 feet) tall, and made of thick metal plates. The second fence, behind the first one, reaches 4.5 meters (15 feet). The top is angled inward, with barbed wire at the top. In some areas, there is a smaller chain-link fence behind the second one. In between the fences is "no-man's land," an area that the U.S. Border Patrol monitors with bright lights, armored trucks, and cameras.

All the border fortification is intended to reduce illegal immigration to the United States from Mexico. Most immigrants who cross the U.S.-Mexico border illegally flee extreme poverty in Mexico. Others have come through Mexico from Central and South America.

Crossing the border near Tijuana-San Diego is dangerous. The crosses in this photograph represent the hundreds of men, women, and children who have died in the area while trying to reach the United States.

Most immigrant deaths in the Tijuana-San Diego area are due to exposure. Exposure is simply being exposed to severe weather. The Sonoran Desert receives very little precipitation. It is very hot during the day, with few trees to provide shade. The desert is also very cold at night. Immigrants who are not prepared with adequate water, protection from the sun, and warm clothing are at risk for dehydration, heat stroke, or hypothermia.

The phrase "Ni Una Muerte Mas! Reforma Ya!" means "Not one more death! Reform now!" It is the slogan of Border Angels, a nonprofit organization focused on American immigration reform and reducing the number of immigrant deaths in the Tijuana-San Diego area.