What does a seismologist study?

Earthquakes, and by extension the inside of the planet - because seismic waves are the only tool that reaches it. Nobody has drilled more than about twelve kilometres down, so everything known about the mantle and core is inferred from how waves change speed and direction passing through material of different density.

That is how the core was found. Some seismic waves cannot travel through liquid at all, which produces a shadow zone on the far side of the Earth from a quake and proved the outer core is molten. In 1936 the Danish seismologist Inge Lehmann noticed waves arriving inside that shadow where none should be, and concluded there must be a solid inner core reflecting them - a discovery made entirely from arrival times on paper records.

The field also does work nobody set out to do. Seismic networks distinguish earthquakes from explosions, which is how nuclear tests are detected and how the test ban treaty is verified. And during the Covid lockdowns of 2020, seismologists worldwide recorded a drop in high-frequency background noise of up to fifty per cent - the sound of traffic and industry disappearing, measurable in the ground.