Valve's Steam page currently lists a second mystery game alongside Deadlock, sending Half-Life 3 theorists into another frenzy of speculation

The Half-Life 3 speculation mill has been running hot over the last few months. G-Man's voice actor Mike Shapiro rang in the new year by posting a video in which he refers to "unexpected surprises" in the staggering voice of Half-Life's dimension-hopping villain. Before that, evidence that Valve was working on a "Project White Sands" was revealed on an actor's website—White Sands referring to an area of New Mexico where Black Mesa was supposedly situated...
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