Resonance Cascade
From Combine OverWiki, the English Half-Life Wikia
A Resonance Cascade was the cause of the Black Mesa Incident, allowing various Xenian species to escape to Earth, who then immediately started killing the first humans they saw.
The Resonance Cascade is also the event that brought the Combine's attention to Earth, and they used it to bring their forces to Earth in ever greater numbers, as discussed by Dr Kleiner in Episode One.
Judging by conversations in the start of the original Half Life, the scientists at Black Mesa knew of the possibility of a cascade but did not comprehend what it might do.
