The giant of light spotted in Antartica. The rumour that Adam was confined deep underground below NERV HQ turned out to be false information. The giant underground is in fact Lilith.
The real Adam is the embryo-looking object brought by Kaji from Germany. This object was created by human when they restored Adam back to the embryonic state after they woke it up with S^2 Theory.
This process seemed to be what had caused the Second Impact. Moreover, it was stated in Genesis of the Old Testament in Bible that Adam was the first human created by God based on His image. There are many theories as to what the word "Adam" means. "Earth", "Red" and "Create" have been put forward as possible theories.