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Known alternately as Gray Monday, the Blackout saw unknown perpetrators disable some 75-80 percent of the Hyperpulse Generator network in simultaneous attacks and acts of sabotage, effectively crippling interstellar communications.
On Sunday, 7 August 3132 many HPG stations in the Inner Sphere, the Clan Occupation Zones and the Periphery were taken down by what appeared to be some sort of system-virus which caused the HPG core to overload and burn itself out with little collateral damage. A few HPG stations didn't burn out but suffered different, sometimes bizarre failures which also discount a virus being the cause.
A number of stations (at one point referred to as "more modern ones") appeared to be immune to whatever affected the rest, but most of these were taken out in a combination of internal attacks of sabotage and swift and efficient military strikes concurrently with the burnout of the others. With the saboteurs either escaping or dying before interrogation, and the surprise of the military assaults, nobody was able to prevent the attacks or learn who was behind the Blackout. The sole piece of identifying information was a strange insignia worn by some of the attackers: a snake coiled around a sword and set against a blood-red disk.
As a result of the Blackout, interstellar communication by and large collapsed, and was reduced to relaying messages via JumpShip in "pony express"-style. The communications blackout had widespread and far reaching ramifications, plunging the Inner Sphere into chaos. Civil unrest grew when it became apparent that the Blackout would not be quick or easy to fix.
Despite the considerable differences between Clan and ComStar HPG protocols and years spent by ComStar's programmers trawling through centuries of kludged-together code updates and patches nothing was found, and even newly manufactured cores showed the exact same symptoms before burning out themselves. ComStar, whose primary reason for existence was the operation and maintenance of the HPG network, threw everything at the problem, but save for briefly restoring the HPG on Wyatt in 3135, nothing has worked, threatening the organisation with irrelevancy, bankruptcy and total collapse.
It has been speculated that the appellation "Gray Monday" for a date that is actually a Sunday comes from the fact that the full impact of the event was not felt until the following day (Monday, 8 August 3132).
Only a handful of HPG stations throughout the Inner Sphere survived. The HPG's ability to send messages to places up to 50 light years away considerably boosted the strategic importance of the worlds with functional HPGs (a working HPG is required to send messages, but not to receive them as HPGs effectively "jump" radio waves which can be received with regular radio equipment). Known worlds with working HPGs after the Blackout include:
- Achernar 
- Devil's Rock
- Gacrux 
- Genoa 
- Hsien 
- Imbros III 
- Irian 
- Kessel 
- Menkar 
- Millungera (temporarily; HPG fix did not last)
- New Aragon 
- Northwind 
- Palmyra (destroyed or irreparably damaged during the Battle of Palmyra)
- Terra 
- Thorin 
- Towne (uncertain due to conflicting information)
- Tsamma (possibly; noted as an 'AFFS hub')
- In the BattleCorps PDF edition of the novel A Bonfire of Worlds, the date of Gray Monday is given as 1 August (instead of 7 August) which, being a Monday, may originally have been the correct date. However, according to the Line Developer, the date is corrected to 7 August in a proof of the novel which is in line with virtually all other Dark Age era publications (save one novel by Blaine Pardoe).
- confirmed by Line Developer Herbert A. Beas II in an official chat on 24 February 2013; see also this article's Talk page
- Era Report: 3145, p. 12 "History and Review - The Fall of Darkness - What about the Clans?"
- By Line Developer Herbert A. Beas II; see Bibliography section
- A Call to Arms
- Era Digest: Dark Age, p. 4 "Functional Republic HPGs as of 01 January 3135"
- By Temptations and by War
- Fire at Will
- Flight of the Falcon
- Fire at Will; restarted by ComStar during timeframe of novel, but A Bonfire of Worlds mentions that it didn't last
- Era Report: 3145, p. 34
- A Call to Arms, Fortress Republic
- Target of Opportunity
- Functional HPG available according to LinkNet articles "Swordsworn, Highlanders Representatives on Towne", dated 01/18/3133; "Jurik: Centauri Lancers Coming to Towne", dated 08/03/3133; Blood of the Isle while LinkNet articles "HPG Working on Achernar?", dated 07/26/3133, and "Message From Achernar", dated 08/02/3133, plus several other INN articles suggest otherwise
- A Bonfire of Worlds, The book indicates it's still operating in 3143.
- Official Line Developer ruling on the correct date, including explanation on why a Sunday would be called "Gray Monday". (Also copied to this article's Talk page for archiving and reference.)
- Era Report: 3145