Objection 1. It would seem that this fire is not beneath the earth. For it is said of the damned (Job 18:18), "And God shall remove him out of the globe [Douay: 'world']." Therefore the fire whereby the damned will be punished is not beneath the earth but outside the globe.
Objection 2. Further, nothing violent or accidental can be everlasting. But this fire will be in hell for ever. Therefore it will be there, not by force but naturally. Now fire cannot be under the earth save by violence. Therefore the fire of hell is not beneath the earth.
Objection 3. Further, after the day of judgment the bodies of all the damned will be tormented in hell. Now those bodies will fill a place. Consequently, since the multitude of the damned will be exceeding great, for "the number of fools is infinite" (Ecclesiastes 1:15), the space containing that fire must also be exceeding great. But it would seem unreasonable to say that there is so great a hollow within the earth, since all the parts of the earth naturally tend to the center. Therefore that fire will not be beneath the earth.
Objection 4. Further, "By what things a man sinneth, by the same also he is tormented" (Wisdom 11:17). But the wicked have sinned on the earth. Therefore the fire that punishes them should not be under the earth.
On the contrary, It is written (Isaiah 14:9): "Hell below was in an uproar to meet Thee at Thy coming." Therefore the fire of hell is beneath us.
Further, Gregory says (Dial. iv): "I see not what hinders us from believing that hell is beneath the earth."
Further, a gloss on Jonah 2:4, "Thou hast cast me forth... into the heart of the sea," says, "i.e. into hell," and in the Gospel (Matthew 12:40) the words "in the heart of the earth" have the same sense, for as the heart is in the middle of an animal, so is hell supposed to be in the middle of the earth.
I answer that, As Augustine says (De Civ. Dei xv, 16), "I am of opinion that no one knows in what part of the world hell is situated, unless the Spirit of God has revealed this to some one." Wherefore Gregory (Dial. iv) having been questioned on this point answers: "About this matter I dare not give a rash decision. For some have deemed hell to be in some part of the earth's surface; others think it to be beneath the earth." He shows the latter opinion to be the more probable for two reasons. First from the very meaning of the word. These are his words: "If we call it the nether regions (infernus [The Latin for 'hell']), for the reason that it is beneath us [inferius], what earth is in relation to heaven, such should be hell in relation to earth." Secondly, from the words of Revelation 5:3: "No man was able, neither in heaven, nor on earth, nor under the earth, to open the book": where the words "in heaven" refer to the angels, "on earth" to men living in the body, and "under the earth" to souls in hell. Augustine too (Gen. ad lit. xii, 34) seems to indicate two reasons for the congruity of hell being under the earth. One is that "whereas the souls of the departed sinned through love of the flesh, they should be treated as the dead flesh is wont to be treated, by being buried beneath the earth." The other is that heaviness is to the body what sorrow is to the spirit, and joy (of spirit) is as lightness (of body). Wherefore "just as in reference to the body, all the heavier things are beneath the others, if they be placed in order of gravity, so in reference to the spirit, the lower place is occupied by whatever is more sorrowful"; and thus even as the empyrean is a fitting place for the joy of the elect, so the lowest part of the earth is a fitting place for the sorrow of the damned. Nor does it signify that Augustine (De Civ. Dei xv, 16) says that "hell is stated or believed to be under the earth," because he withdraws this (Retract. ii, 29) where he says: "Methinks I should have said that hell is beneath the earth, rather than have given the reason why it is stated or believed to be under the earth." However, some philosophers have maintained that hell is situated beneath the terrestrial orb, but above the surface of the earth, on that part which is opposite to us. This seems to have been the meaning of Isidore when he asserted that "the sun and the moon will stop in the place wherein they were created, lest the wicked should enjoy this light in the midst of their torments." But this is no argument, if we assert that hell is under the earth. We have already stated how these words may be explained (91, 2).
Pythagoras held the place of punishment to be in a fiery sphere situated, according to him, in the middle of the whole world: and he called it the prison-house of Jupiter as Aristotle relates (De Coelo et Mundo ii). It is, however, more in keeping with Scripture to say that it is beneath the earth.
Reply to Objection 1. The words of Job, "God shall remove him out of the globe," refer to the surface of the earth ["De orbe terrarum," which might be rendered "from the land of the living."], i.e. from this world. This is how Gregory expounds it (Moral. xiv) where he says: "He is removed from the globe when, at the coming of the heavenly judge, he is taken away from this world wherein he now prides himself in his wickedness." Nor does globe here signify the universe, as though the place of punishment were outside the whole universe.
Reply to Objection 2. Fire continues in that place for all eternity by the ordering of Divine justice although according to its nature an element cannot last for ever outside its own place, especially if things were to remain in this state of generation and corruption. The fire there will be of the very greatest heat, because its heat will be all gathered together from all parts, through being surrounded on all sides by the cold of the earth.
Reply to Objection 3. Hell will never lack sufficient room to admit the bodies of the damned: since hell is accounted one of the three things that "never are satisfied" (Proverbs 30:15-16). Nor is it unreasonable that God's power should maintain within the bowels of the earth a hollow great enough to contain all the bodies of the damned.
Reply to Objection 4. It does not follow of necessity that "by what things a man sinneth, by the same also he is tormented," except as regards the principal instruments of sin: for as much as man having sinned in soul and body will be punished in both. But it does not follow that a man will be punished in the very place where he sinned, because the place due to the damned is other from that due to wayfarers. We may also reply that these words refer to the punishments inflicted on man on the way: according as each sin has its corresponding punishment, since "inordinate love is its own punishment," as Augustine states (Confess. i, 12).
Source: Summa Theologica (New Advent)