Pathfinder: Shadow of Chernobyl
In 1359 a Second Disaster occurred at the Chernobyl iron mines.
Whilst there was once again a great magical explosion like in the First Disaster, this time the fabric of reality seemed to become twisted as well.
The Second Disaster expanded the breadth of the Zone to its present limits of about 75km across and created the Anomalies, Artefacts and Mutants that you can find there today. The zone has been slowly, but steadily increasing in size since.
After initial forays by the military, the entire Zone was cordoned off to prevent further disasters.
Information on artefacts, anomalies and the effects of the magical fallout that were gathered in the few months before the cordon would eventually lead to the development of potions of Magical Cleansing. These potions, if used regularly, allow individuals to stave off the adverse effects of magical infusion whilst exploring the Zone.
In 1365, equipped with ample supplies of this new potion, the Lithuanian Army began staging outposts within the Zone itself hoping to recover artefacts and eventually reclaim the mine itself. Hampered by the mutated fauna of the zone, deadly anomalies and a healthy dose of superstition they have made little progress.