Reineke, director of the Colibrî Center for Human Rights, holds the personal effects of unidentified border crossers.

For those attempting to cross the border, the passage can be perilous.

Wilson, a volunteer with Humane Borders, leaves water in the desert for migrants and leads search missions when they are reported missing.

Water is salvation in the desert. Water is survival. Pastor Mike Wilson believes access to water is the most fundamental human right other than air. He leaves gallons of water in the Sonoran Desert for migrant travelers crossing the border into the United States.

Diaz, Franco, Castillo and Nava-Cervantes are members of Alianza Nacional de Campesinas, which advocates for female farmworkers rights, including prompt wage payment and protection from sexual harassment.

Undocumented immigrants lack workplace protections.

Garcia says she came to the U.S. from Mexico illegally two decades ago and married a U.S. citizen, raising three children. She says she returned to Mexico in 2012 to see her dying mother and that when she tried to re-enter the U.S. six months later, she was arrested and deported.

The arrest and detention of undocumented immigrants has ballooned over the past two decades.

Ruiz, pictured with her daughter Mercedez, 19, and grandson Christopher, 1, is a legal resident who was arrested in 2011 and detained for seven months while she fought deportation based on a 2002 misdemeanor drug conviction.

Liu says she was trafficked to the U.S. at age 18 by smugglers who promised her employment but then demanded $98,000. She received a visa as a trafficking victim in November 2011, according to her lawyer.

Sylvain, who says he left Haiti for the U.S. at age 7 and became a legal resident, now faces deportation for allegedly forging his cousin’s signature on traffic tickets in 2001. He poses with his wife Joeddy, 30, and their son Joland, who are both U.S. citizens.

Lopez Cash says her 13-year-old son Omar, pictured, tried to cross the U.S.-Mexico border to join his family in July 2010. Almost three years later, his remains were found in the Arizona desert.

Kim and her brother Peter, 27, arrived in the U.S. from South Korea with their parents when Kim was 9, she says. Their father became a permanent resident through marriage to a U.S. citizen. Peter became a citizen, but Kim did not because she was over 21, she says.

Since 2009, about two million undocumented immigrants have been blocked at the border or removed from the U.S. after being detained elsewhere.

Joseph is a green card holder and decorated veteran of the Gulf War whom U.S. immigration officials detained for three years while they unsuccessfully sought to deport him after his conviction for transporting weapons. His son Japeri wears his uniform.

Garcia Gamez, left, a Venezuelan, and his partner of 20 years, Santiago Ortiz, a Puerto Rican, registered as domestic partners in New York City in 1993. They were married in Connecticut in 2011, and Garcia Gamez gained permanent resident status in 2013, he says.

Personal effects found with migrant remains, like this watch, are cataloged at the Pima County Office of the Medical Examiner in Arizona.

At the Pima County Office of the Medical Examiner, the remains of migrants found near the border are inspected and stored.

Velazquez says her partner was arrested last year in Maricopa County, Ariz., for working with false documents. She says he spent more than 10 months in immigration detention and missed the birth of his son Aaron, pictured.

Velazquez's mother says her partner was arrested last year in Maricopa County, Ariz., for working with false documents. She says he spent more than 10 months in immigration detention and missed the birth of his son.

In 2010, at the San Ysidro port of entry near San Diego, California, Anastasio Hernandez Rojas was beaten and shocked with a stun gun by Border Patrol agents and died three days later. He had been deported a few months earlier and was trying to reenter the United States and rejoin to his wife, Maria Puga, and their 7-year-old twins, Daniela and Daniel, pictured here near the wall where Rojas was beaten. Puga, who has successfully demanded an investigation into her husband's death, says “I want my kids to grow up peacefully, not with rancor. That is why I continue to search for justice.”

Herminia Gallego Lopez, her two daughters Rosy Griselda Rojas Gallego (age 19) and Fatima Rojas Gallego (age 12), and her sister-in-law Margarita Rojas, wait in Nogales, Mexico, after being deported from the United States.

Alma D. Isais Aguilar, a Catholic nun, works with the Kino Border Initiative, a humanitarian organization that helps to provide food and shelter to migrants in the border town of Nogales, Mexico. Sister Alma reports that in the past, most migrants were people heading north for the first time, but she now increasingly sees long-term residents of the United States who have been deported and are trying to return to their families.

An unidentified man sits at the Casa del Migrante, a migrant shelter in Tijuana, Mexico.

A shrine to the Virgin Mary at the Casa del Migrante, a migrant shelter in Tijuana, Mexico.

The border wall between the United States and Mexico extends 300 feet into the ocean at Tijuana, Mexico.

Jacqueline Rayos (R), and Mayra Lovatos, both 11, are cousins and US citizens. But Jacqueline's mother and Mayra's aunt, Guadalupe Garcia de Rayos, is undocumented. She was arrested in 2008 in a raid conducted by Sheriff Joe Arpaio of Maricopa County and eventually put into deportation proceedings. After a public campaign by the immigrant advocacy group Puente, Guadalupe was granted a one-year stay of removal. Jacqueline and Mayra were photographed after a protest organized by Puente Arizona in front of Immigration and Customs Enforcement headquarters in Phoenix, Arizona, on July 29, 2013, a few days after Guadalupe's stay was granted. On February 8, 2017 -- during her annual appointment with immigration officials -- agents arrested Guadalupe and began procedures to send her back to Mexico, a country she has not seen since she left 21 years ago.