According to the New Testament, hell is a place of fire (Matt. 3.12, 5.22, 13.40-42, 50, 18.8, 25.41; Mark 9.43, 48; Luke 16.24; Heb. 10.27; 2 Thess. 1.8; cf. Jude 7; Rev. 19.20, 20.10). This idea can be traced back to the pre-Christian era , and ultimately to the motif of fire associated with the eschatological "Day of Yahweh", which survived into the apocalyptic movement (see here) and the later books of the Old Testament (see Isaiah 66.24; Sirach 7.19).
Hell is also described as a place of darkness (Jude 13; 2 Peter 2.4, 17; cf. Matt. 22.13, 25.30). In addition, it is said to be infested with worms (Mark 9.48 - see also, in the Old Testament, Isaiah 66.24 and Sirach 7.19).
How much of this is to be taken literally, according to the theologians? Augustine noted that the fire and the worms were interpreted in different ways: some thought that both were physical, some that both were spiritual (City of God, 20.22). He himself thought that it was "absurd to suppose that either body or soul will escape pain" (21.9).
Aquinas likewise reported that there had been many opinions on the nature of hell-fire (Summa Theologiae, Suppl., 97.5). He thought (97.5-6) that the fire of hell was physical, like fire on earth (in this, he followed Gregory the Great: see Dialogues, 4.29). He also thought that the darkness would be literal, such that nothing in hell would be seen clearly but "such things as are able to bring anguish to the heart" would be seen dimly (97.4). On the other hand, he argued that the worms had to be metaphorical because no bodies of animals would remain in the next world (97.2).
Writers who regarded hell-fire as metaphorical included Ambrose (Commentary on Luke, 14), the 16th century theologian Ambrosius Catharinus, and Origen (De Principiis, 2.10.4), who had this to say:
[E]very sinner kindles for himself the flame of his own fire, and is not plunged into some fire which has been already kindled by another, or was in existence before himself. Of this fire the fuel and food are our sins.... And I think that, as abundance of food, and provisions of a contrary kind and amount, breed fevers in the body, and fevers, too, of different sorts and duration, according to the proportion in which the collected poison supplies material and fuel for disease... so, when the soul has gathered together a multitude of evil works, and an abundance of sins against itself, at a suitable time all that assembly of evils boils up to punishment, and is set on fire to chastisements; when the mind itself, or conscience, receiving by divine power into the memory all those things of which it had stamped on itself certain signs and forms at the moment of sinning, will see a kind of history, as it were, of all the foul, and shameful, and unholy deeds which it has done, exposed before its eyes: then is the conscience itself harassed, and, pierced by its own goads, becomes an accuser and a witness against itself.
Where is hell?
Whole books have been written on this question (Giovanni Patuzzi, De sede inferni (1763); Jacob Gretser, De subterraneis animarum receptaculis (1595)).
The Israelites seem to have believed that the realm of the dead was beneath the earth (Num. 16.31-33, 1 Kgs 28.13, Sirach 24.45, 46.23). There are passages in the New Testament which likewise imply that hell is located underground (Matt. 12.40, Eph. 4.9, Phil. 2.10, Rev. 5.3, 12.9).
Gregory the Great inclined to the view that hell was underneath the earth, but acknowledged that the question was a disputed one: "Some thought hell is somewhere on earth; others believe it is under the earth" (Dialogues, 4.42). Augustine changed his mind on the issue, eventually coming round to the view that hell was subterranean (Retractions, 2.29). Aquinas wrote that "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." (Summa Theologiae, Suppl. 97.7).
The 1910 Catholic Encyclopedia had this to say:
[A]ll kinds of conjectures have been made; it has been suggested that hell is situated on some far island of the sea, or at the two poles of the earth; Swinden, an Englishman of the eighteenth century, fancied it was in the sun; some assigned it to the moon, others to Mars; others placed it beyond the confines of the universe....However, the catechism of Pope St Pius X, published in 1908, defined hell as a "state" rather than a place. In the period prior to Vatican II, Ludwig Ott's classic Fundamentals of Catholic Dogma (1952) did not take a position on whether hell was a place or a state.
The Church has decided nothing on this subject; hence we may say hell is a definite place; but where it is, we do not know.
Is hell eternal?
Yes, according to the New Testament (Matt. 3.12, 18.8, 25.46; Mark 9.43, 48; cf. Jude 7; Rev. 20.10). The eternality of damnation was subsequently affirmed in the Athanasian Creed, by the Fourth Lateran Council (Firmiter) and by the Council of Trent (sess. 6, ch. 14; sess. 14, can. 5).
The eternity of hell is a notoriously challenging doctrine, and, as Augustine put it, "some, indeed very many, make moan over the eternal punishment, and perpetual, unintermitted torments of the lost, and say they do not believe it shall be so" (Enchiridion, 112).
Wilhelm and Scannell's Manual of Catholic Theology (1908) states:
The teaching of the Fathers on the eternity of Hell is almost unanimous. St. Clement of Rome, St. Justin Martyr, Theophilus, Tertullian, Minucius Felix, St. Irenaeus (Contra Haer., iv. 28), St. Cyprian (Ad Demetr., 24, 25), and Hippolytus - to mention only the early Fathers - all speak of "eternal punishments," "unquenchable fire," "eternal fire," "torments without end" (see Petavius, De Angelis, lib. iii. c. 8). The great Origen, it is true, held that all men, and even the devils, would be saved at last (De Princ., i. 6; In Josu., Hom. viii.); and his teaching to some extent influenced the opinions of St. Gregory of Nyssa (Or. Cat., 26), St. Gregory of Nazianzum, St. Ambrose, and St. Jerome (In Is., xiv. 20), see Petavius (l.c., cap. 7).Theophylact put the matter succinctly: "Origen is babbling nonsense when he says that there will be an end to hell" (Commentary on Matthew, 3.12). Aquinas too taught that punishment in hell would be eternal, albeit the damned would see the saved until judgement day (Summa Theologiae, Suppl. 98.9, 99.1-4).
[Note:] St. Gregory of Nazianzum hoped that sinners would not be punished for ever; St. Jerome that at least sinners who were Catholics would not be so punished. St. Ambrose's opinion was that men not devils may be purified and restored even after condemnation at the judgment.
A distinction may be made between Origen's theory that even the devils would be saved and the less radical view that only humans would be set free from hell (cf. Augustine, City of God, 21.17-18).
A few writers, including the poet Prudentius and the writer of the 2nd century Apocalypse of Peter, thought that God relaxed the punishments of the damned on certain occasions, such as Sundays.