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Unless you have the parts to build a new supercomputer, this place isn't going to be safe much longer. Nice job on that, by the way.

Safe being relative here Stuck inside with a psychotic rampaging AI that gassed the rest of the facility...

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Clearly, you were in the stasis chamber during the Black Mesa incident, and GLaDOS has been managing to keep the various Xen baddies out of Aperture labs the whole time.

I'd guess at sometime between HL1 and HL2. The shot where you wake up after blowing up GLaDOS shows it be fairly normal looking. No sign of combine or a massive citadel in the background.