A lot of people think that Frigga is the biological mother of Thor - especially given that the recent live action movies didn't suggest anything to the contrary - but the fact is that, in the comic books at least, his biological mother is actually Gaea. Based on the Gaia of Greek mythology and Joro of Norse mythology, Gaea was a primeval Earth goddess, who had infused her life essence into those of all Earth's living beings. She was the embodiment of the spirit of life, growth, harvest and renewal on Earth and was extremely powerful. It was her relationship with Odin, prior to him meeting Frigga, that led to the birth of the God of Thunder and, given some of his lesser known powers over Earth and the elements, it makes sense when you think about it.
