Horrific as S-21 was, Choeung Ek was even worse: this was the killing field where most of S-21's prisoners were actually executed. Every night, covered trucks would show up, full of the condemned. From there, they would be marched to the edge of a mass grave, and the slaying would begin. Bullets were scarce, so the executioners used whatever they could get their hands on: machetes, car axles, sharpened bamboo sticks, palm fronds, rocks. Music was played over loudspeakers so the local villagers wouldn't hear the screams of those being bludgeoned and hacked to death.