Objection 1. It would seem that the limbo of hell is the same as the hell of the damned. For Christ is said to have "bitten" [Allusion to Hosea 13:14] hell, but not to have swallowed it, because He took some from thence but not all. Now He would not be said to have "bitten" hell if those whom He set free were not part of the multitude shut up in hell. Therefore since those whom He set free were shut up in hell, the same were shut up in limbo and in hell. Therefore limbo is either the same as hell, or is a part of hell.
Objection 2. Further, in the Creed Christ is said to have descended into hell. But he did not descend save to the limbo of the Fathers. Therefore the limbo of the Fathers is the same as hell.
Objection 3. Further, it is written (Job 17:16): "All that I have shall go down into the deepest hell [Douay: 'pit']." Now since Job was a holy and just man, he went down to limbo. Therefore limbo is the same as the deepest hell.
On the contrary, In hell there is no redemption [Office of the Dead, Resp. vii]. But the saints were redeemed from limbo. Therefore limbo is not the same as hell.
Further, Augustine says (Gen. ad lit. xii): "I do not see how we can believe that the rest which Lazarus received was in hell." Now the soul of Lazarus went down into limbo. Therefore limbo is not the same as hell.
I answer that, The abodes of souls after death may be distinguished in two ways; either as to their situation, or as to the quality of the places, inasmuch as souls are punished or rewarded in certain places. Accordingly if we consider the limbo of the Fathers and hell in respect of the aforesaid quality of the places, there is no doubt that they are distinct, both because in hell there is sensible punishment, which was not in the limbo of the Fathers, and because in hell there is eternal punishment, whereas the saints were detained but temporally in the limbo of the Fathers. On the other hand, if we consider them as to the situation of the place, it is probable that hell and limbo are the same place, or that they are continuous as it were yet so that some higher part of hell be called the limbo of the Fathers. For those who are in hell receive diverse punishments according to the diversity of their guilt, so that those who are condemned are consigned to darker and deeper parts of hell according as they have been guilty of graver sins, and consequently the holy Fathers in whom there was the least amount of sin were consigned to a higher and less darksome part than all those who were condemned to punishment.
Reply to Objection 1. When Christ, by His descent, delivered the Fathers from limbo, He is said to have "bitten" hell and to have descended into hell, in so far as hell and limbo are the same as to situation.
This suffices for the Reply to the Second Objection.
Reply to Objection 3. Job descended, not to the hell of the damned, but to the limbo of the Fathers. The latter is called the deepest place not in reference to the places of punishment, but in comparison with other places, as including all penal places under one head. Again we may reply with Augustine (Gen. ad lit. xii): who says of Jacob: "When Jacob said to his sons, 'You will bring down my grey hairs with sorrow to hell,' he seems to have feared most, lest he should be troubled with so great a sorrow as to obtain, not the rest of good men, but the hell of sinners." The saying of Job may be expounded in the same way, as being the utterance of one in fear, rather than an assertion.
Source: Summa Theologica (New Advent)