Ka-boom! On January 6, machines around the world recorded a clatter. North Korean officials said, “The first H-bomb test was successfully conducted in [North Korea].” Was that claim true? Scientists would help find out.

Experts studied the January jolt. Was it an earthquake? Was it an airplane’s sonic boom . . . or something much worse?

Hydrogen bombs, or H-bombs, are much more powerful than atomic bombs (A-bombs). How strong? You need an A-bomb to start an H-bomb. That’s how strong.

Analysts knew there was only one way to tell about the bomb. They needed to study the data. The U.S. Geological Survey reported in. It uses seismography. Seismography works like this: Sensors are set up all around the globe. The sensors are part of machines that measure movement. Each machine draws out the results of the movement at its location. That image looks like a bunch of squiggly lines. Earthquakes, storms, volcanic eruptions—all those things get measured.

Scientists know how to read those lines. They watch the readings. They may be able to warn people of upcoming danger. For instance, sometimes they can predict that a volcano could erupt. Tremors below ground can suggest an earthquake is coming. Often, scientists can tell the difference between manmade movements and natural movements.

The seismic equipment did record movement near North Korea. It occurred about an hour before North Korea announced the bomb test. Paul Richards studies seismic signals. He said the tremors didn’t seem strong enough to be an H-bomb.

Scientists don’t know for sure what happened in North Korea in January. Most believe the movement wasn’t from a powerful H-bomb. But even if it was a bomb, the seismic readings helped put people’s minds at ease. The news wasn’t as bad as it could have been.