Resonance cascade

Black Mesa was revealed to be conducting top-secret research into various fields, such as teleportation and experimental weapons research. During this research, they managed to teleport to Xen, and in doing so they are always bringing back crystals and creatures to test on in their research chambers.

At the begining of Half Life, one such crystal, provided by the engimatic character of the G-Man, is put through an anti-mass spectrometer and causes a resonance cascade, tearing the spacetime continuum allowing creatures from Xen to randomly teleport to Black Mesa.