The Wages of Fear

Frenchmen Mario and Jo, Dutchman Bimba and Italian Luigi are stuck in the isolated town of Las Piedras. Surrounded by desert, the town is linked to the outside world only by a small airport, but the airfare is beyond the means of the men. There is little opportunity for employment aside from the American corporation that dominates the town, Southern Oil Company (SOC), which operates the nearby oil fields and owns a walled compound within the town. SOC is suspected of unethical practices such as exploiting local workers and taking the law into its own hands, but the townspeople's dependence upon it is such that they suffer in silence.
Mario is a sarcastic Corsican playboy, who treats his devoted lover, Linda, with disdain. Jo is an aging ex-gangster who just recently found himself stranded in the town. Bimba is an intense, quiet individual whose father was murdered by the Nazis, and who himself worked for three years in a salt mine. Luigi, Mario's roommate, is a jovial, hardworking individual, who has just learned that he is dying from cement dust in his lungs. Mario befriends Jo due to their common background of having lived in Paris, but a rift develops between Jo and the other cantina regulars because of his combative, arrogant personality.
A massive fire erupts at one of the SOC oil fields. The only way to extinguish the flames and cap the well is an explosion caused by nitroglycerine. With short notice and lack of proper equipment, it must be transported within jerrycans placed in two large trucks from the SOC headquarters, 300 miles away. Due to the poor condition of the roads and the highly volatile nature of nitroglycerine, the job is considered too dangerous for the unionized SOC employees.
The company foreman, Bill O'Brien, recruits truck drivers from the local community. Despite the dangers, many of the locals volunteer, lured by the high pay: US$2,000 per driver. This is a fortune to them, and the money is seen by some as the only way out of their dead-end lives. The pool of applicants is narrowed down to four handpicked drivers: Mario, Bimba and Luigi are chosen, along with a German named Smerloff. Smerloff fails to appear on the appointed day, so Jo, who knows O' Brien from his bootlegging days, is substituted in his place. The other drivers suspect that Jo murdered Smerloff in order to facilitate his own hiring.
Jo and Mario transport the nitroglycerin in one vehicle; Luigi and Bimba in the other, with thirty minutes separating them in order to limit potential casualties. The drivers are forced to deal with a series of physical and mental obstacles, including a stretch of extremely rough road called "the washboard", a construction barricade that forces them to teeter around a rotten platform above a precipice, and a boulder blocking the road. Jo finds that his nerves are not what they used to be, and the others confront Jo about his increasing cowardice. Finally, Luigi and Bimba's truck explodes without warning, killing them both.
Mario and Jo arrive at the scene of the explosion only to find a large crater rapidly filling with oil from a pipeline ruptured in the blast. Jo exits the vehicle to help Mario navigate through the oil-filled crater. The truck, however, is in danger of becoming bogged down and during their frantic attempts to prevent it from getting stuck, Mario runs over Jo. Although the vehicle is ultimately freed from the muck, Jo is mortally wounded. On their arrival at the oil field, Mario and Jo are hailed as heroes, but Jo is dead and Mario collapses from exhaustion. Upon his recovery, Mario heads home in the same truck, now freed of its dangerous cargo. He collects double the wages following his friends' deaths, and refuses the appointed chauffeur offered by SOC. Mario jubilantly drives down a mountain road, while a party is being held at the cantina back in town where Mario's friends eagerly await his arrival. Mario swerves recklessly and intentionally, having cheated death so many times on the same road. He takes one corner too fast and plunges through the guardrail to his death. Linda, dancing in the cantina, appears to faint.

Question: What do Mario and Jo discover at the scene of the explosion?
Answer: A large crater