Spoilers ahead
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Catalogued reference data from ARDA's R&D department, regarding previously discovered Anomalies, which - at the time the game takes place - are now extinct or exceedingly rare.
These in-game documents can be found scattered throughout the Zone, and then stored inside of the Fax Machine at the Auto Shop.
Anomaly Studies #1: Squealer
ARDA Cat. Ref. 1.22
One of the earliest consistently reported Anomalies, this mimicked many artificial noises made in its presence. A tiny, burrowing, bottle-shaped Anomaly, this would surface and imitate engine noises, radio static, or, in particular, sirens.
Studying these was difficult as those approaching them suffered immediate nausea and dizziness. Sightings dropped to zero soon after the first Crawlers manifested.
Anomaly Studies #2: Hat Man
ARDA Cat. Ref. 3.63
While this Anomaly cannot strictly be described as humanoid, it has nevertheless gained the moniker "Hat Man" and a slew of distant sightings describe a first impression of a tall person in a hat and coat.
Sightings are consistent in their description of a tall, thin shape moving at or just above ground level, with a broad, low, hat-like top above an almost cone-like tapering lower structure. This can give the impression of a wide-shouldered figure.
No sightings of this Anomaly have occurred since June 1964. However, at this time, many Zone personnel began to report extremely consistent dreams of something similar entering their homes.
Anomaly Studies #3: Antenna Chewer
ARDA Cat. Ref. 2.14
Sighted consistently between late 1959 and early 1961, this Anomaly was attracted to any transmitters broadcasting at particular shortwave bands. About the size of a bird, roughly spherical, chrome-like and able to fly through unknown means, this Anomaly would latch itself to broadcast antennae and begin to dissolve, or perhaps consume, the metal.
Stubborn and difficult to dislodge, this Anomaly began to disappear when Zone personnel began using different frequencies.
Anomaly Studies #4: Mosquito
ARDA Cat. Ref. 2.79
Briefly a widespread blight, this tiny, flying Anomaly would bore into oil or gasoline tanks to seemingly drink from them. Rust-colored, about the size of a horsefly and fast-moving, these were common through the summer of 1962.
Though tougher than an insect, most could be swatted or stomped on. Sightings are now extremely rare, but occasionally still logged.
Anomaly Studies #5: Icicle
ARDA Cat. Ref. 4.11
These long, thin, ice-like Anomalies were found in clusters on trees and overhangs throughout the early '70s. Responsible for the majority of Anomaly-related injuries at that time, they would cause serious contact burns and were easily dislodged by wind of movement.
They were present during all types of weather and, if disturbed or manipulated, would quickly dissolve away into nothingness. None have been reported since 1975.
Anomaly Studies #6: Match Head
ARDA Cat. Ref. 3.51
Localized patches of this Anomaly appeared throughout fringe areas of the Zone between 1967 and 1968. Thin, waist-height, grass-like structures would spring up overnight. These were topped with variously-coloured bulbs that would frequently combust with the slightest friction.
This Anomaly was aggressively culled whenever it was found and has not been reported since December 1968.
Anomaly Studies #7: Ink Sac
ARDA Cat. Ref. 4.85
Waist-height and one of the few apparently benign Anomalies reported, these were frequently founding hiding in warm, dry places during cold or wet weather. This included under vehicles and by heating vents.
Bulbous and resembling puffball mushrooms, they would squirt dark liquid when disturbed and roll away rapidly. Rarely sighted after 1976, but occasionally still reported.