Who are all "under the earth”?

Revelation 5:13 (ESV)

13 And I heard every creature in heaven and on earth and under the earth and in the sea, and all that is in them, saying, “To him who sits on the throne and to the Lamb be blessing and honor and glory and might forever and ever!”

This refers to the realm of the dead (Daniel 12:2; Isaiah 26:19), sometimes called Sheol or Hades. John heard even those already in torment on account of their rejection of the Lord join in praise the worthy Lamb! “Under the earth” would also include the inhabitants of the bottomless pit (Revelation 9:1), namely, the demons with their master Satan.