The Skaniska Incident took place in the Russian Steppes sometime in around 2004. Over the course of five days, thirteen out of fifty UNIT soldiers died during operational manoeuvres while protecting the local population, despite wearing Standard DL/3 class X-hazard chemical survival suits to protect themselves from chemical pollution. (PROSE: UNIT's Position on the Skaniska Incident)
Behind the scenes
The Russian Steppes seems to be a reference to the Pontic–Caspian steppe.
Information from invalid sources
However, the operation was noted as being a success on the news area of the UNIT website, and Captain Martin Bury, the leader of the operation, as promoted to Major and placed in charge of the Geneva B Company. (NOTVALID: UNIT News)
According to one account, however, the Skaniska Incident was series of events wherein a unknown entity crashed into the Russian steppes, a tundra, and caused the deaths of the seven UNIT soldiers sent to investigate. (NOTVALID: The UNIT Sourcebook)