Valve is known for making quality first-person shooters: The Half-Life series redefined how players think about the FPS genre, while titles like Team Fortress and Left 4 Dead took team-oriented multiplayer action to a whole new level. But among Valve's stable of popular shooters, one game stands alone: the mind-bending puzzler, Portal. A small-scale experiment that was squeezed into 2007's The Orange Box alongside some powerful heavyweights, the love Portal received from fans was beyond anything Valve could have imagined, winning it a full-fledged, standalone sequel.

Steel yourself for another round of ...whatever it is that Valve's doing with Portal PC, as a second mystery update for the game just arrived via Steam. The only clue note:

# Added valuable asset retrieval

The first mystery update, released Monday, added various radio transmissions to the 2007 game--leading to Morse code, hidden images and even a secret Apeture Science BBS--all the while prompting speculation that a Portal 2 and/or Half-Life 2: Episode Three announcement is imminent.

Update: Gamers are reporting that the ending to the game has been changed.

Portal's eerie symbolism was ramped up another notch this week when Valve announced a patch that has gamers playing what seems to be an alternate reality game within the platformer.