An earthquake is what happens when the ground shakes suddenly because of movement deep inside of the Earth.

The Earth’s outer layer is broken into huge slabs of rock called tectonic plates, which move slowly and sometimes get stuck along cracks in the crust called faults.

When the rocks finally slip or break, they release a burst of energy that travels through the ground as shaking.

The underground point where an earthquake starts is called the focus, and the point on the surface directly above it is the epicenter.

Scientists record earthquakes using a tool called a seismograph and describe their size using magnitude.

The Richter scale is the most familiar name for this, though scientists now commonly use the moment magnitude scale instead.

Each whole number on the scale represents about 32 times more energy, so a magnitude 6 earthquake releases far more energy than a magnitude 5, and a magnitude 7.2 quake produces roughly 32 times the energy of a magnitude 6.2 one.

Before modern electronics, scientists built spring-pendulum seismometers to measure this, including one that weighed 15 tons, and a three-story-tall version still operates in Mexico City today.

Most earthquakes are too small for people to notice, but larger ones can damage homes, roads, bridges, and other infrastructure, and can also trigger landslides, fires, and tsunamis.

How much damage occurs depends on the earthquake’s magnitude, its depth, its distance from populated areas, building quality, and the type of ground it travels through.

Scientists cannot predict exactly when a specific earthquake will strike, but they can study faults and past activity to identify higher-risk areas.

During shaking, people are taught to drop, cover, and hold on: get low, take cover under sturdy furniture, and hold on until it stops.

Earthquakes are extremely common worldwide, with roughly 500,000 detectable each year, about 100,000 of them strong enough to feel, and only around 100 causing real damage.

Most occur along the Pacific Ring of Fire, the belt of active faults and volcanoes that surrounds the Pacific Ocean.

In the United States, Alaska sees more earthquakes than any other state, often experiencing a magnitude 7.0 quake every year, while Florida and North Dakota experience the fewest, and from 1975 to 1995 those two states plus Iowa and Wisconsin recorded none at all.

Some of the most extreme earthquakes on record stand out for very different reasons.

The largest ever recorded struck Chile in 1960 at magnitude 9.5.

The deadliest known earthquake hit central China in 1556, killing an estimated 830,000 people, many of whom lived in caves carved from soft rock that collapsed on them.

Alaska’s 1964 Good Friday earthquake lasted about seven minutes and was felt across 1.3 million square kilometers, while the 2004 Indian Ocean earthquake lasted nearly ten minutes, the longest on record, and triggered a devastating tsunami.

A major earthquake can release hundreds of times more energy than the atomic bomb dropped on Hiroshima.

Earthquakes can also produce strange effects far from where they start.

During Mexico’s 1985 earthquake, water reportedly sloshed out of swimming pools in Tucson, Arizona, more than 2,000 kilometers away.

This sloshing motion in pools, lakes, or enclosed bays is called a seiche, and it can continue for hours after the shaking itself has stopped.

Some scientists also believe animals may be able to sense weak tremors or electrical signals before an earthquake hits, though this remains debated.

The idea of huge cracks suddenly opening up and swallowing people or animals whole is also mostly fiction, even though a popular legend claims a cow fell into one during the 1906 San Francisco earthquake.

California’s San Andreas Fault isn’t a single crack either, but a fault zone stretching more than 800 miles and up to 10 miles deep, and it’s why Los Angeles and San Francisco are slowly creeping toward each other at about 1.5 inches a year, a pace that would bring them together in roughly 15 million years.

Earthquake history also includes some notably strange and even funny moments.

Much of the destruction after the 1906 San Francisco earthquake actually came from the fires that followed rather than the shaking itself, and one displaced resident reportedly labeled his refugee tent the “House of Mirth.”

The French actress Sarah Bernhardt performed in Berkeley just weeks after that quake and is said to have remarked that the ruins were more impressive than any she’d seen in Greece or Rome, while a champagne salesman donated an entire boxcar of champagne to victims to considerable press fanfare.

After a minor 2011 earthquake in Virginia, West Coasters mocked East Coasters online with a photo captioned “DC Earthquake Devastation” that showed nothing more than a single flipped lawn chair, and one San Francisco resident tweeted that a 5.8 quake was nothing compared to what the West Coast has “for breakfast,” even as some East Coasters reportedly fled their buildings, the opposite of the recommended response.

In 1990, a former NASA consultant with a background in zoology rather than seismology predicted a massive earthquake would strike the Midwest on December 3; it never happened, despite people throwing “earthquake parties” and at least one bar offering a “shaken, not stirred” drink special.

The word “tsunami” itself comes from Japanese for “harbor wave,” a rather modest name for something that can be so destructive.