What Does "Under The Earth" In Philippians 2:10 Refer To?

Php 2:10 that at the name of Jesus every knee should bow, of those in heaven, and of those on earth, and of those under the earth,

"Under the earth" could simply mean the waters under the earth.

Ex 20:4 "You shall not make for yourself a carved image, or any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth;

If this is the meaning, then the apostle Paul was simply saying that the entire creation, every sphere of it (the heavens, the earth and those in the waters under the earth) that God has made will someday worship and own the Lord Jesus Christ as Lord.

Ps 148:1 Praise the LORD! Praise the LORD from the heavens; Praise Him in the heights!
2 Praise Him, all His angels; Praise Him, all His hosts!
3 Praise Him, sun and moon; Praise Him, all you stars of light!
4 Praise Him, you heavens of heavens, And you waters above the heavens!
5 Let them praise the name of the LORD, For He commanded and they were created.
6 He also established them forever and ever; He made a decree which shall not pass away.
7 Praise the LORD from the earth, You great sea creatures and all the depths;
8 Fire and hail, snow and clouds; Stormy wind, fulfilling His word;
9 Mountains and all hills; Fruitful trees and all cedars;
10 Beasts and all cattle; Creeping things and flying fowl;
11 Kings of the earth and all peoples; Princes and all judges of the earth;
12 Both young men and maidens; Old men and children.
13 Let them praise the name of the LORD, For His name alone is exalted; His glory is above the earth and heaven.
14 And He has exalted the horn of His people, The praise of all His saints-Of the children of Israel, A people near to Him. Praise the LORD!

However, the apostle Paul seems to refer only to created intelligences that God has made.

Php 2:10 that at the name of Jesus every knee should bow, of those in heaven, and of those on earth, and of those under the earth,
11 and that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father.

Paul then is referring to angels and saved sinners up there heaven, all the people here on earth and all those who are under the earth or the underworld. Paul is not referring to universal salvation since the Bible makes it very clear that not all will be saved. Rather, Paul is referring to universal subjugation.

Much like what happened during the Roman empire. Everyone under the empire bowed down and owned the lordship of the emperors, even their enemies. Those who have renewed and regenerated hearts will willingly own Him as Lord. While the unsaved and the infernal demonic spirits will do so out of defeat.

Verses 10 and 11 are only partially fulfilled right now. It will fully and completely be fulfilled when He comes back again to reign here on earth and on into eternity.

"Under the earth" or the underworld is the place of the departed spirits of human beings and demonic spirits. Where exactly this is, we don't know. All we know is that it is under the earth. Elsewhere in the Bible, it is called the Pit or Sheol or Hades.

Isa 38:18 For Sheol cannot thank You, Death cannot praise You; Those who go down to the pit cannot hope for Your truth.

 Isa 14:15 Yet you shall be brought down to Sheol, To the lowest depths of the Pit.

Eze 26:20 'then I will bring you down with those who descend into the Pit, to the people of old, and I will make you dwell in the lowest part of the earth, in places desolate from antiquity, with those who go down to the Pit, so that you may never be inhabited; and I shall establish glory in the land of the living.

Ps 49:9 That he should continue to live eternally, And not see the Pit.

 Re 9:1 Then the fifth angel sounded: And I saw a star fallen from heaven to the earth. To him was given the key to the bottomless pit.

 Re 20:3 and he cast him into the bottomless pit, and shut him up, and set a seal on him, so that he should deceive the nations no more till the thousand years were finished. But after these things he must be released for a little while.

1Pe 3:19 by whom also He went and preached to the spirits in prison