The 2nd Khartiia Corps of the Ukrainian National Guard announced carrying out a special operation underground, appearing to use an underground pipeline network. The operation was dubbed "Veles" and was carried out in complete informational silence. The results will be released to the public tomorrow, October 22. The trailer video they released doesn't reveal many clues. Presumably it was an underground pipeline network. There are several of these near the frontline, including one that connects Kupyansk with the city of Urazove in Belgorod Oblast. A couple of months ago I was told by a source that a significant portion of the fighting for Kupyansk city was taking place underground, but this operation seems to be much bigger than simple frontline clashes. There are other tunnel complexes near Pokrovsk and Kursk Oblast which could also have been used.