To find out what the Earth is like inside, geologists have studied rocks collected in mines and deep boreholes, materials emitted by volcanoes, and the behavior of seismic waves during earthquakes.  
In 1936 the geologist Inge Lehmann observed that, at a depth of about 5,150 km, compressional P-waves from earthquakes underwent a marked increase in their propagation velocity through the Earth's core.  
From this he deduced the existence of a solid inner core surrounded by the liquid outer core, which is surrounded by the mantle and the mantle by the crust. 
- Draw the internal structure of the earth showing the boundaries of the different layers, indicating the depth and discontinuities.