As the whole world continues to explore the deep recesses of Aperture Science in Portal 2, its more compact predecessor has quietly reached an impressive benchmark. Valve has reported that the original Portal has now reached over 4 million sales worldwide - and believe it or not, that's excluding Steam purchases. In other words, 4 million retail copies of Portal have been picked up off store shelves.

The runaway smash hit of the Orange Box in 2007 was undoubtedly Portal. Its beginnings as a small indie production known as Narbacular Drop are now nearly unrecognizable in comparison to the franchise that now dominates both the gaming industry and pop culture (cakes, lies, cubes and such). It is very impressive that the relatively short and genre-bending prequel to Portal 2 is still racking up enough sales to push it over the 4 million mark.

Of course, its more robust and feature-rich sequel is not to be outdone by this news. According to various sources, Portal 2 sold an estimated 675,000 copies on the day of its release. Nary a gamer was willing to miss out on what is sure to be one of the games of the year come the end of 2011.

Unlike the nearly flawless Orange Box addition however, Portal 2 has garnered a fair amount of criticism since yesterday, falling prey to some unhappy gamers who stormed Metacritic to lower the user score significantly, bottoming out near a 5.0. Loyal fans have helped push the score upwards to the mid-7s since. The complaints of day one DLC and an inordinately short campaign have mostly been resolved by the reasonable folk of everywhere else but Metacritic.

Portal 2 received our best score of the year so far, and if its any indication of day one purchases, Portal 2 should reach 4 million sales much quicker than Portal.

Do you think that Portal 2 will be as much of a fixture in everyone's mind as Portal before it, or will sales peter out after the Portal fans have had their fill?

Portal and Portal 2 are currently available for PC, Mac, Xbox 360 and PS3.

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Sources: Gamespot UK, VGChartz