The Circus Anomaly is a location that only exists in S.T.A.L.K.E.R.: Call of Pripyat, in Zaton.
Overview
The player will first come into contact with the Circus Anomaly after he has completed his mission to get onto the plateau with Noah. After he is done investigating the crashed helicopter, he can climb down the slope to the Circus Anomaly, which is a circular area with two Comet anomalies going around it and several Burner anomalies in the center. It is not advisable to explore here if you have broken armor, because investigating this anomaly is not required to complete the main plot in the game, but if you are looking for artifacts to sell (or to hoard) and your armor is in good condition, by all means stroll in with your detector. You will usually find at least one artifact there, although it will likely not be a top a top tier artifact.
Circus Anomaly; do not get caught in the swirling Comet anomalies on the outside