What is the significance behind the skull at the base of a crucifix?
There seems to be two interpretations of the meaning of the skull at the base of the cross, the first is more symbolic and the second is more historical:
The skull represents Adam, the first man, along with original sin. Jesus was sent to Earth to absolve us of our sins through His death. Jesus' blood is washing away our sins by flowing across the skull of Adam and that Jesus is above sin. (Source: http://ricklobs.blogspot.com/2009/03/have-you-ever-noticed-skull-at-base-of.html)
The hill that Jesus was crucified on was called Golgotha, or Skull Hill. This is where Adam was reportedly buried and the Lord was crucified where Adam lay. (Source: http://mysite.pratt.edu/~wburg/paint/p3skull.html) (Source 2: http://en.wikipedia.org/wiki/Golgota)
According to a teaching by Perry Stone, who does much research of ancient writings, the skull of Goliath was possibly buried on that hill, as well as the shape of the hill favoring a skull, Which also represents the head of the serpent, which bruised the heel of Christ, but Christ crushed his head. I'm being brief, but there is quite a teaching on this subject. Orthodox scholars would probably be familiar with these same ancient church and rabbinical writings.
The skull is the location of where Adam..The first Human was buried. The hill on which the Messiah was crucified was called in Hebrew Gulgatha...The Place of The skull...the whole purpose of the Jews to return to Jerusalem was for the messiah to be crucified there, to fulfill the prophesies of the old Testament. Christs blood fell over Adams bones.