You absolutely must read this article HERE . . . .

In case you dont get this refrence, in Half Life 1 you play as Gordon Freeman (a scientist at the top secret Black Mesa Research Facility) who travels to work one day and inadvertently causes a ‘resonance cascade’ . . . thus tearing a hole in space time that allows aliens from another world (another dimension ?) to come through and start attacking us (as aliens often do). 

Look at the wiki page to get a more detailed picture if you’re interested . . . HERE