The Black Mesa Incident, referred to once as the Lambda Incident, was a catastrophic event caused by an experiment carried out at the Black Mesa Research Facility. The experiment triggered a Resonance Cascade, which allowed numerous extraterrestrial to teleport into the facility and wreak havoc upon its personnel. The ensuing conflict between United States military forces and the alien creatures resulted in the complete destruction of the facility and the death of almost all of its employees. The Black Mesa Incident would ultimately lead to the invasion of Earth by the Combine.