Through reflection & refraction of seismic waves, we �see�
Earth is composed of layers with different characteristics & compositions
Crust - regions of low seismic velocity; 5-7 km thick in oceans & 35-70 km thick in continents
Moho - discontinuity at base of crust; discovered early in the 20th century
Mantle - region of high seismic velocity below crust; dense substrate on which crust floats