X-18 is a secret laboratory located underneath the Dark Valley. It has a reputation as being one of the most terrifying (and most dangerous) locations in the series.
X-18 is a laboratory that was used to study the effects of psychic radiation (based on the informational field, noosphere) on living cells using Kaymanov emitters. It was set up in the wake of the 1986 disaster. Mutations of living beings following exposure to these emissions is meticulously noted.
In one of the last experiments, mutations involved hair loss, aggression, etc. Eventually, subjects stabilized, with only 10% of their hair remaining and substantial growth of claws and teeth. Bursts of seemingly random activity were present, but not directed at their peers, leading to speculation on an increase in intellect. Thus, the second stage of the experiment with a new modulation of psi-emissions was authorized. Security incidents took place, as the aggressive subjects were prone to causing damage when prompted to by researchers or security guards. In one heinous instance, two dozen test subjects broke out and vandalized the main experiment area and forcing the researchers to lock down the lower levels. The security team had to respond with force, suffering minor wounds. The final experiment concerned the use of new psi-emissions on test subjects, which ended with their brains roasted. The experiment was considered a success and led to experimentation on larger creatures. 
The end of the laboratory came when, after receiving too much radiation, an experiment broke out and destroyed most of X-18 (it did this by opening all the other containers and releasing the previous experiments). The surviving staff members locked themselves out of the main hall by a series of large coded doors. Eventually, their experiments reached them through the piping, and killed them off. Later, Stalkers began venturing into the laboratory for any treasures that may remain there. Few survived, many died from shock, while others were at the mercy of the Poltergeists.
Borov, the bandit leader in Shadow of Chernobyl, discovered that the door may not have been locked, he then proceeded to lock it, keeping what treasure may be inside safe for him to collect at a later date. Borov was always too afraid to enter the lab after locking it, thus it remained locked until the Marked One unlocked it, using the keycard he recovered from Borov after killing him.
Mutants / Anomalies
- One Pseudogiant.
- Two Snorks.
- Six Poltergeists.
- One Pyrogeist in the room with the documents.
- Burner anomalies.
- A Comet anomaly.
- A Fruit Punch anomaly.
- "Documents from X18, Part 3", that the Barkeep requires.
- AKM-74/2 fitted with a Grenade launcher.
- Two Medkits.
- One Monolith Suit in a locker. Potentially one other in a stash safe.
- Varying amounts of ammunition.
- While the location of X-18 is beneath Dark Valley, on the PDA it's depicted as an above ground location close to the Red Forest. This is because no map exists of the area.
- If you revisit the lab after you took the documents, more Snorks can spawn here, and the Pseudogiant can spawn again too.
- "The report details the study of biological samples sent from lab X18. The samples were subjected to informational field effects of varying intensities. The report meticulously describes the mutation of living beings following exposure to emissions.\nThese documents must not fall into anyone else's hands, as they would be of great interest to USS analysts." - Report on study of samples
- "Hell... Lab X18 wasn't fun...and its current residents...I didn't think things like that existed. I found some documents...I didn't understand much of what was written in there except that they contained technical specifications for some components for Kaimanov emitters. What's interesting is that the parts were destined for another lab with the codename X16 and it's probably a secret one too."- Strelok's note in the PDA
- "A week ago it was decided to increase the intensity of radiation on the test subjects. No changes were detected in the first few days. After several days strange metamorphoses started to occur: hair loss, heightened aggressiveness. Several subjects were literally trampled by their peers." - The beginning of the experiment
- "Subjects' condition has stabilized. The amount of hair is now 10% of the original amount. Their claws and teeth have grown considerably. Bursts of activity exhibit seemingly random behaviour and are not directed at their peers. Senior lab technician A.N. Burnov speculated that this may indicate an increase in the subjects' intellect. It was decided to test the new modulation of psi-emissions on the subjects." - The second stage of the experiment
- "Chief of Security: Yesterday security guard G.P. Glebov tossed a piece of meat to the subjects just for fun. They tore apart and devoured it in mere seconds. Subsequently several individuals tried to break out of the cells. Because of this, lab technicians were unable to obtain the necessary test samples. Although G.P. Glebov's actions were ill-considered, I propose that maintenance personnel be equipped with airguns for adequate protection in the event of emergencies." - Security report
- "Chief of Security: Yesterday the experiment almost got out of control. Two dozen test subjects broke out and vandalised two laboratory rooms. Fortunately, the research personnel were able to lock themselves up upstairs. We were ordered to clear the main laboratory room. In the course of the operation, three guards suffered bites. Hopefully, they will be OK. I wish the experiment is soon over." - Attack on the personnel
- "Yesterday we delivered a new modulation of psi-emissions to the test subjects. It appeared as though their brains simply got roasted. I can hardly imagine how this emission would affect humans. We have tested every type of emission and the results match those obtained in other laboratories. The experiment can now be considered complete. Presumably, in one week's time, we will be ready to conduct the experiment on larger creatures." - The last stage of the experiment
- The state of the laboratory, as well as the presence of dead bodies of researchers are clear indication that the experiments spun out of control.