20th Century incursion
The Anomaly first opened up in the early 20th Century, and was found by John Mortimer's scientific team. A flock of Terror Birds came through the Anomaly and killed Mortimer's team, and at some point after this, the Anomaly closed.
21st Century incursion
In the early 21st Century, the Anomaly reopened and a flock of Terror Birds came through again. When the Birds attacked the ARC team, Danny Quinn used a recording of the Birds' distress call to draw them back to the Anomaly. In response to the call, the Terror Birds returned through the Anomaly, which closed minutes later. (Episode 3.6)
- The scene where the Anomaly opens appears in James Lester's flashback recap of the three previous series in Episode 4.1.