The Killing Fields

In the Cambodian capital, Phnom Penh, during May 1973, the Cambodian national army is fighting a civil war with the Khmer Rouge, a result of the Vietnam War overspilling that countryâs borders. Dith Pran, a Cambodian journalist and interpreter for The New York Times, awaits the arrival of reporter Sydney Schanberg at the city's airport but leaves suddenly. Schanberg takes a cab to his hotel where he meets up with Al Rockoff (John Malkovich). Pran meets Schanberg later and tells him that an incident has occurred in a town, Neak Leung; allegedly, an American B-52 has bombed the town.
Schanberg and Pran go to Neak Leung where they find that the town has been bombed. Schanberg and Pran are arrested when they try to photograph the execution of two Khmer Rouge operatives. They are eventually released and Schanberg is furious when the international press corps arrives with the U.S. Army.
Two years later, in 1975, the Phnom Penh embassies are being evacuated in anticipation of the arrival of the Khmer Rouge. Schanberg secures evacuation for Pran, his wife and their four children. However, Pran insists that he would stay behind to help Schanberg.
The Khmer Rouge move into the capital, ostensibly in peace. During a parade through the city, Schanberg meets Rockoff. They are later met by a detachment of the Khmer Rouge, who immediately arrest them. The group is taken through the city to a back alley where prisoners are being held and executed. Pran, unharmed because he is a Cambodian civilian, negotiates to spare the lives of his friends. They do not leave Phnom Penh, but instead retreat to the French embassy.
Informed that the Khmer Rouge have ordered all Cambodian citizens in the embassy to be handed over and fearing the embassy will be overrun, the embassies comply. Knowing that Pran will be imprisoned or killed, Rockoff and fellow photographer Jon Swain (Julian Sands) of The Sunday Times try to forge a British passport for Pran; the deception fails when the image of Pran on the passport photo fades to nothing, as they lack adequate photographic developer. Pran is turned over to the Khmer Rouge and is forced to live under their totalitarian regime.
Several months after returning to New York City, Schanberg is in the midst of a personal campaign to locate Pran. In Cambodia, Pran has become a forced labourer under the Khmer Rouge's "Year Zero" policy, a return to the agrarian ways of the past. Pran is also forced to attend propagandist classes where many undergo re-education. As intellectuals are made to disappear, Pran feigns simple-mindedness. Eventually, he tries to escape, but is recaptured. Before he is found by members of the Khmer Rouge, he slips into a muddy cesspool filled with rotting human corpses; in doing so, he stumbles upon the infamous killing fields of the Pol Pot regime, where as many as 2Â million Cambodian citizens were murdered.
In 1976 Sydney Schanberg is awarded a Pulitzer prize for his coverage of the Cambodian conflict. At the acceptance dinner he tells the audience that half the recognition for the award belongs to Pran. Later in the restroom, he is confronted by Rockoff, who harshly accuses him of not doing enough to locate Pran and for using his friend to win the award. Schanberg defends his efforts, saying that he has contacted every humanitarian relief agency possible in the time since Pran's disappearance. Rockoff suggests that Schanberg subtly pressured Pran to remain in Cambodia because Pran was so vital to Schanberg's work. This accusation hits close to home, and Schanberg begins to wonder whether he put his own self-interest ahead of Pran's safety. He finally confesses that Pran "stayed because I wanted him to stay".
Pran is assigned to the leader of a different prison compound, a man named Phat, and charged mostly with tending to his little boy. Pran continues his self-imposed discipline of behaving as an uneducated peasant, despite several of Phatâs attempts to trick him into revealing his knowledge of both French and English. Phat begins to trust Pran and asks him to take ward of his son in the event that he is killed. The Khmer Rouge are now engaged in a border war with Vietnam. The conflict reaches Pran's region and a battle ensues between the Khmer Rouge of the compound and two jets sent to destroy the camp. After the skirmish has ended, Pran discovers that Phat's son has American money and a map leading to safety. When Phat tries to stop the younger Khmer Rouge officers from killing several of his comrades, he is ignominiously shot.
In the confusion, Pran escapes with four other prisoners and they begin a long trek through the jungle with Phatâs young son. The group later splits and three of them head in a different direction; Pran continues following the map with the fourth man. However, Pranâs companion activates a hidden land mine while holding the boy. As Pran pleads with the man to give him the boy, the mine goes off, killing the pair. Pran mourns for a time and continues on. One day he crests the escarpment of the DÃ¢ngrÃªk Mountains and sees a Red Cross camp near the border of Thailand.
The scene shifts to Schanberg calling Pran's family with the news that Pran is alive and safe. Soon after, Schanberg travels to the Red Cross camp and is reunited with Pran. He asks Pran to forgive him; Pran answers, with a smile, "Nothing to forgive, Sydney", as the two embrace and John Lennon's song "Imagine" is heard in the background. The following credit scene gives short-sentenced conclusions on the characters' lives after the events of the movie.

Question: What did they trek through?
Answer: The jungle