In the year King Uzziah died, I saw the Lord seated on a high and lofty throne, with the train of his garment filling the temple. Seraphim were stationed above; each of them had six wings: with two they veiled their faces, with two they veiled their feet, and with two they hovered aloft. They cried one to the other, “Holy, holy, holy is the LORD of hosts! All the earth is filled with his glory!” At the sound of that cry, the frame of the door shook and the house was filled with smoke. (Is 6:1-6)
https://bible.usccb.org/bible/readings/071324.cfm
The Call of the Prophet Isaiah begins with this vision of the Lord and the seraphim. The song of the seraphim is echoed in the Sanctus (Holy, Holy, Holy) of the Eucharist. In one sense the liturgy on earth unites with the liturgy of heaven in one chorus of endless praise.