(Chorus of Furies): "I can see before me
the earth's navel, which has taken
bloodshed on itself, a ghastly defilement to have." (166-167)
The "earth's navel"? What the heck are they talking about? So glad you asked. Actually, the earth's navel—bellybutton if you prefer—was a stone placed in the temple of Apollo at Delphi. This stone is commonly called in English by its Greek name, "omphalos," which is just the Greek word for bellybutton. You can see a picture of the omphalos of Delphi here.
Why Delphi? Because the people of Delphi considered themselves the center of everything, that's why. Seriously, we're not kidding.
So, if the omphalos of Delphi was the center of the world, what's that have to do with gender? Well, check out the first quotation from this section. There, you'll see that the Earth is imagined as a female goddess. How do these pieces fit together? Think about it, Orestes, dude who killed his mother, is lying on the omphalos or bellybutton-stone of the Earth, which reminds us of an umbilical cord, which reminds us that the Earth is the mother of us all.
Basically, the way the Greeks thought of the gender of the Earth gives Aeschylus an opportunity to work a really cool metaphor into the Furies' song. By touching the omphalos of Delphi with the blood of his own mother, Orestes is defiling the entire earth, the entire idea of motherhood, and all of nature.