As Leila has guessed, there was a huge fight in the mountain ranges they are in. The military outpost has detected the movements of the two overlords months prior before they finally fought head on. Thinking that the fight will not happen any time soon as most cases of overlords fighting, they were all caught off guard.
In the records they had over the years, whenever they detected some anomalies with the powerful beasts in the area it took at least a year or so before they brought destruction to the whole place. It was the reason why they still continued the planned practicals with a peace of mind as it was only a month since they found the signs. The scheduled mayhem is still far away and therefore still safe for their students to get in there.
