Stalker: Shadow of Chernobyl → Research Institute "Agroprom" (Part 1)
The research institute area is controlled by soldiers—you'll constantly encounter patrols on the road. Beyond the institute's gates, stalkers battle the military. Help Mole's group, and he'll show you the entrance to the sewer system where Strelok once stored his valuables.
This dungeon will also lead you to the research center, where on the third floor there is a suitcase that the merchant needs.
Note: In the center of a small lake in the northwest of the area, you can talk to a character—a non-aggressive soldier—who will tell you that the only way to quietly enter the research center is through the dungeon. Before you venture into the dungeon, be sure to find at least one silent pistol—you'll need stealth very soon. Let's go down to the dungeon.