Seismology is the scientific study of earthquakes and the movement of waves through the Earth. The field also includes studies of variants such as seaquakes, causes such as volcanoes and plate tectonics in general, and offshoot phenomena such as tsunami.
Earthquakes (and other earth movements) produce different types of seismic waves. These waves travel through rock, and provide an effective way to "see" events and structures deep in the Earth.
One of the earliest important discoveries was that the center of the Earth is liquid. Pressure waves pass through the core. Waves that shake side-to-side, requiring a rigid material, do not.
The process of mapping subsurface features is a specialty called seismography. Seismic waves produced by explosions have been used to map salt domes and other oil-bearing rocks, faults (cracks in deep rock), rock types, and long-buried giant meteor craters. For example, Chicxulub, the meteor that is believed to have killed the dinosaurs, was localized to central america by analyzing ejecta in the cretaceous boundary, and then physically proven to exist using seismic maps from oil exploration.
Using seismic tomography with earthquake waves, the interior of the Earth has been completely mapped to a resolution of several hundred kilometers. This process has enabled scientists to identify convection cells, magma plumes and other large features of the inner Earth.
Seismographs also effectively discover unusual, otherwise unobserved phenomena such as large meteors striking uninhabited ocean, or underground nuclear tests. Ocean meteor strikes as large as ten kilotons (of TNT, effective explosive force) have been reported.
In 2002, using high-resolution digital seismic records, some scientists have reported discovering several point sources of seismic noise that enter the Earth, move through it in a straight line at supersonic speeds, and then leave it. These anomalies may be dense dark matter, magnetic monopoles or quantum black holes passing through the Earth.