A Great Earthquake
There is a theory that the opening of the sixth seal is a reference to the rapture.
Vine’s definition of G4578 — "a shaking, a shock," from seio, "to move to and fro, to shake," chiefly with the idea of concussion (Eng., "seismic," "seismology," "seismometry"), is used
Matt. 27:54 Now when the centurion, and they that were with him, watching Jesus, saw the earthquake, G4578 and those things that were done, they feared greatly, saying, Truly this was the Son of God.
The rapture will cause the whole world to be shaken and shocked and declare that Jesus was the Son of God.
Matt 28:2 And, behold, there was a great earthquake: G4578 for the angel of the Lord descended from heaven, and came and rolled back the stone from the door, and sat upon it.
The resurrection is associated with a great earthquake.
Acts 16:26 And suddenly there was a great earthquake, G4578 so that the foundations of the prison were shaken: and immediately all the doors were opened, and every one's bands were loosed.
The rapture happens "suddenly" and it shakes the foundation of the prison and it causes all the doors for the gospel to be opened and all the bands keeping the believers quiet will be loosed.
Rev 6:12 And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; G4578 and the sun became black as sackcloth of hair, and the moon became as blood;
Each seal causes something that is a hidden mystery to be revealed when it is opened. The first seal revealed how Satan uses the illusion of being a savior on a white horse coming to save us with a rainbow depicting equity embracing all. For thousands of years people were manipulated by these evil rulers but now that is all being unsealed. The second seal reveals how these Satanic rulers use war to move their agenda with no regard to the lives of their subjects. The third seal shows how they can use famine to manipulate people into doing what they want, the fourth seal shows how they plagues to manipulate people and the fifth seal shows all the whistleblowers, many of whom have been killed, who witnessed against these evil rulers.
So then, what is the sixth seal? Could it be a revelation that the faithful witness of the church is over, and now the whole world sees that the Bible is the truth and Jesus is the Son of God and the church were those who were faithful witnesses to the Lord?