The Resonance Cascade is a fictional scientific disaster that occurred at the Black Mesa Research Center in the video game "Half-Life." It occurred after Gordon Freeman placed the Xen crystal sample into Black Mesa's anti-mass spectrometer.
It is discovered by characters in the "Half-Life" world that the Xen crystal sample would resonate under observation. The anti-mass spectrometer caused enough resonance to tear open dimensional barriers between Earth and Xen, a place where dimensions intersect, allowing beings from the other side onto Earth. Therefore, this event is the direct cause of the Black Mesa Incident, worldwide portal storms, the Seven Hour War and the subversion of Earth by an extradimensional empire called the Combine.