Millions of people across a US state simultaneously dropped to the floor, clambered under tables and covered their heads in a dramatic earthquake drill.
Experts say "the big one" is inevitable and long overdue in the Los Angeles area and so the annual drill is taken extremely seriously.
The Great Shakeout saw entire school districts, colleges, work places and public buildings, all taking part in the exercise which has grown every years since it started in 2008.
In keeping with the drill's quake-survival message, participants are urged to "drop, cover and hold" - meaning get down on hands and knees, cover their heads and necks under a sturdy piece of furniture and hang on until the hypothetical shaking stops.
One of the larger gatherings was held at the Natural History Museum in Exposition Park near downtown Los Angeles.
It was based on the premise of a magnitude 7.8 quake striking the southern end of the San Andreas Fault, a subterranean chasm between two massive plates of the Earth's crust that extends hundreds of miles across California.
Great Shakeout exercise designer Erik Franco said: "The exercise we did today serves as an example to every single corporate office, agency or organisation, even households and schools, of how they should have a team that's prepared, ready and snaps to action after an earthquake happens to go ahead and assess their exit routes, to assemble on the outside and to take care of each other afterwards.
"So this serves as an example for everybody to be prepared and to train and to drill, wherever they might be."
The earliest recorded earthquake in California happened in 1769 and was documented by Spanish explorers.
One of the worst happened in 1933 when the Long Beach earthquake happened in a densely populated area.
It damaged and destroyed a huge number of public buildings and schools in Long Beach and Los Anegeles.
Hospitals were damaged in the San Fernando earthquake .
These two events led to laws being changed so properties had to be built to withstand earthquakes.