In the video game, Half-life, what event started the Half-life universe as we know today?

The Resonance Cascade is significant because it is the event that triggered the events in the Half-Life universe as we know it today. It occurred when an experiment at the Black Mesa Research Facility went wrong and caused a dimensional rift, leading to the invasion of the alien species known as the Combine. This catastrophic event not only unleashed dangerous creatures upon Earth but also set the stage for the subsequent events that take place in the Half-Life series, including the rise of the protagonist, Gordon Freeman, and his ongoing battle against the forces threatening humanity.