The Agroprom Underground is a collection of tunnels and rooms beneath the Agroprom Research Institute and connects to two places in that area - west of the first installation (outside the broken wall) and inside the walls at the second installation. The Underground is where Strelok's hideout is located.
The Underground is vital to the main storyline as the Marked One must travel here to advance the quest to discover his origins. The player will first visit Strelok's hideout during the early parts of the game after rescuing Mole, and a second time after visiting Lab X-16, where they are given the key to the Hotel by Doc. There is some interesting loot in the hideout, such as a Bandit Jacket, Sunrise suit and the unique Fast-shooting Akm 74/2.
The tunnel is full of anomalies, but also of artifacts. The tunnel directly south of the entrance can be a gold mine for artifacts should one go down there between midnight and 5am. The usual booty is plenty of Sparklers, a few Flashes, and if the Electros are generous, up to two Moonlights.
The area is populated by a group of ten soldiers, as well as Bandits during the first trip there. Friar can also be found there. There is a Bloodsucker often spawning in the large room just before the first soldiers, and a Controller will intercept the player.
The Agroprom Underground returns in Clear Sky. Scar will be sent there by General Krylov to flood parts of the underground to kill mutants invading the Agroprom. The player is then able to explore a new part of the underground, that is later flooded and made inaccessible. After fighting a Controller in a large room, Scar floods and escapes the underground before entering Strelok's stash and finding a PDA here, confirming Strelok attempted to return to the center of the Zone.
There is a tank close to an exit with a leak of now solidified slag. It looks similar to the "Elephant's Foot" corium leak structure below reactor 4 in the Chernobyl Nuclear Power Plant.
It is best to time the visit so that the Agroprom Underground raid finishes a little after midnight (or wait it out Underground until then) because at that time the level spawns artifacts and, moreover, at night the security in the Agroprom Institute is relaxed, making stealing the documents a little easier. A perk of waiting Underground after completing Strelok's stash is that after some time Friar might spawn in the tunnel near the entrance, bringing with him the Storming Obokan. Clearing the level at a relaxed pace can take 3-5 in-game hours.