The Hellgate. After the war in heaven, this is the spot where Lucifer fell. Upon Lucifer's fall from heaven and through earth, a rift was opened, creating a portal to Lucifer's kingdom. This portal was later sealed by the seven ruling Archangel: Gabriel, Raphael, Anahel, Michael, Samael, Sachiel, and Cassiel; when it posed a threat to earths survival. The Grigori (fallen angels living as mortals) dedicated thier lives to trying to reopen the gates to hell once more, and for the last two and a half thousand years they have secretly been gathering what they need to bring the fallen Lucifer back to the thrown of Hell.
This is the Seal used on the unholy ground to Seal Lucifer's powers. It is located deep under ground, under the actual crater. The Grigori built their temple around the Seal, underneath the crater.