Global seismic observations are essential for understanding earthquake mechanisms and for building a three-dimensional picture of Earth's internal structure. Seismic waves generated by large earthquakes, in particular, travel through Earth's deep interior and can be recorded at stations around the globe. This seismological information, together with other geophysical and geological data, can be used to infer Earth's geologic history and also to study its present-day dynamics, such as mantle convection and plate motions. Data quality and quantity increased significantly in the 1960s as seismic stations were installed on continents and islands globally (e.g., World-Wide Standardized Seismograph Network). Another revolution took place in the 1980s when seismic observatories shifted to digital data acquisition and broadband (1 mHz-10 Hz) sensors. This global presence and standard technology enabled scientists to build the first three-dimensional Earth models. The seismological community expects that further refinement of such models will come from filling in the gaps in global coverage through establishing seismic stations in the deep ocean where coverage is poor.