The shadow zone is the area of the earth from angular distances of 104 to 140 degrees that, for a given earthquake, does not receive any direct P waves. The shadow zone results from S waves being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid core.
Through measuring how P and S waves travel through the earth and out the other side, a seismic wave shadow zone was discovered in about 1910. From the lack of S waves and a great slowing of the P wave velocity (by about 40%) it was deduced that the outer core is made of liquid. The shadow zone also defined the diameter of the core.