In 1755, when a deadly earthquake struck the city of Lisbon, no one understood what could cause the ground to shake as it did. Observations of residents at the time indicated that the quake had delivered two distinct types of vibrations. This video segment describes the history of seismology. Topics include the two main types of seismic waves (S- and P-waves), how a seismograph works, and how seismograph readings from three locations can be used to pinpoint the location of an earthquake. There is also discussion of the uses of seismograms (the records produced by the instruments), and the Richter scale for quantifying earthquake magnitude. The segment is three minutes thirty-nine seconds in length. A background essay and discussion questions are included.