In 1755, a deadly earthquake struck the city of Lisbon. At the time, scientists had little understanding of what could cause the ground to shake as it did. Observations of residents indicated that the quake had delivered two distinct types of vibrations. Since that time, scientists have developed and used a device called a seismometer to better understand seismic waves. Today, seismometers help scientists predict earthquakes. This video segment adapted from NOVA describes the history of seismology.