Stalker: Shadow of Chernobyl → Wild Territory
You'll have to make your way to Amber Lake through the Wildlands (directly from the northern checkpoint). This huge station is controlled by mercenaries, and they don't take kindly to outsiders. Soon, you'll witness the mercenaries shoot down a helicopter carrying scientists who were peacefully flying to their bunker. You'll have the opportunity to rescue one of the scientists by escorting them to the lake. This isn't as difficult as it seems—the scientist tries to stay out of trouble.
Beyond the bridge with fire anomalies, you'll encounter the first zombies. They wander slowly, but shoot accurately. Along the way, you'll reach the vicinity of the dried-up Amber Lake.