Is seismology really the only way to measure the thickness of Earth's layers?
Robert Mallet is sometimes referred to as the "father of seismology." He was helped (around 1850) by the newly created science of photography, which enabled him to create seismic "maps" of the earth, at different depths.

He has started life as an iron engineer/manufacturer, but turned his talents to seismology after a number of devastating earthquakes in Europe. His earlier work with iron gave him a background in the use of explosives to "open up" the earth at critical points.

Paleoseismology is a related field that relies on paleontology to form estimates of various properties of the earth.