Some 5 million Californians promise to drop to the ground, cover their heads and hold onto furniture Thursday as if an earthquake had just struck.

Don't worry, it's not real. But because earthquakes are a big threat to California, the state is starring in a mock quake disaster drill billed as the largest in U.S. history.

The exercise is based on a hypothetical magnitude-7.8 temblor that ruptures the southern San Andreas Fault — an event scientists call the "Big One." Such a quake would cause 1,800 deaths and $200 billion in damage, researchers estimate.

The state's previous simulated quake catastrophes were smaller in scale, with the leading actors mainly first responders and cities testing their emergency preparedness. The drill Thursday is more of an ensemble cast, with governments, schools, hospitals, churches, businesses and residents promising to do their part.