The Tunguska - K-T event Anomaly was a gigantic recurring Anomaly linking a Siberian forest in Tunguska, Russia in the present day to a woodland area right near the time of the K-T event (extinction event at the very end of the Cretaceous).
In comparison to other Anomalies, the Tunguska Anomaly was far bigger and more unstable than any other before it; the exact size was not stated but it was lighting the sky at night and could be seen from several miles away. The Anomaly was also extremely recurrant and periodic, opening and closing many times between 1908 and 2007, sometimes between short intervals of no more than a matter of hours - days. Its strength and power was such that when the K-T comet impact was approaching on the Cretaceous side of the Anomaly, the Anomaly impacted on other Anomalies in the present; causing them to become more wild and unstable.
Nick and Helen Cutter both theorised that the reason this Anomaly was so large, recurrant and powerful was because the electromagnetic destruction from meteor impacts had heavily damaged and widened the Anomaly's temporal fault. They also suggested that either: the meteorites which had repeatedly hit directly on the Anomaly site due to being drawn by the Anomaly's magnetic field had been what enlarged and damaged the Anomaly; or alternatively, the Anomaly's unstable and enlarged state could have been caused by the K-T impact which would happen in the near-future on the Cretaceous side sending shockwaves backwards along the time stream, in a predestination paradox. (Extinction Event)
This Anomaly had apparently been regularly opening and closing since at least the early 20th century due to its enlarged and erratic state. It was responsible for the 1908 Tunguska meteorite blast when an incoming asteroid on the Cretaceous side was drawn to the Anomaly, and more, less-destructive repeats of this phenomenon would occur over the following hundred years; two more major asteroid impacts occurred at the Anomaly site, in the early 1960s and circa 2006 respectively, and Koshkin mentioned that there was evidence to suggest there had been up to a dozen lesser meteor impacts. Many Cretaceous creatures - both Dinosaurs and prehistoric Mammals - came through the Anomaly from the Cretaceous to the present, forming a small biome of Cretaceous animals in modern Tunguska's wilderness surrounding the Anomaly site.
After the Tunguska research team who had been assigned to investigate and deal with the creatures in Tunguska kidnapped the Anomaly Research Centre team for help, Nick Cutter, Connor Temple and Abby Maitland successfully tracked the Anomaly down. Cutter and Abby went through into the Cretaceous, along with Koshkin, Rina Suvova, Grisha Bulov and two other soldiers, before the Anomaly temporarily closed and stranded them there.
Once the Anomaly had re-energised enough to fully reopen, Cutter, Abby and the Russians returned through to the present, along with Jenny Lewis, Jake Hemple and the latter's SAS men who had met up with them in the Cretaceous. As the Chicxulub impactor which would cause the K-T Mass Extinction Event was only hours - days away from impact on the Cretaceous side of the Anomaly, both the ARC team and Koshkin's team set about getting the Anomaly sealed, lest the blast from the K-T impact affect the present world as well as the K-T boundary. Despite interference from an unreasonable and skeptical Shvachko, Connor and Koshkin ultimately succeeded in hacking a Russian HERF weapon and firing it at the Anomaly, successfully closing it and saving the modern world from devastation.
Cutter believed that the HERF would have sealed the Anomaly permanently, lthough he advised Koshkin to continuing monitoring the Tunguskan site just in case. (Extinction Event)