In "Book I" of Paradise Lost Milton gives us some very memorable images of hell; many of them allude to the Bible, and others from his own mind. Here are some of them:
"bottomless perdition" (I. 47)
"fiery gulf" (I. 52)
"A dungeon horrible" (I. 61)
"one great furnace flamed, yet from those flames / No light (I. 62-63) My favorite.
"Regions of sorrow, doleful shades, where peace / And rest can never dwell, hope never comes / That come to all" (I. 65-67)
These are incredible images of Milton's hell. The first one alone is astonishing. In the two words "bottomless perdition" Milton gives us a sense of the magnitude of the punishment; it is eternal; this idea is accomplished via the word "bottomless." How does one escape a bottomless anything let alone a "perdition?"
The other that is quite mind-boggling is "flames" that cast "No light." This paradox certainly further enhances the "bottomless perdition" idea, doesn't it? The image reveals just how hopeless it is in hell, for light represents hope; Milton confirms this lost of hope when he says Hell is a place
And rest can never dwell, hope never comes
That comes to all (I. 66-67)
I'm afraid that a complete description would take quite a while (more space than is available here), because it would involve things like describing the mindset of Satan and the fallen angels.
However, you can get a good discussion of some elements of hell in the summary and analysis of Book 1. There and in the poem itself you'll find it mentioned that there is a lake of fire in hell, that it is "horrid," "dismal," a "dungeon," full of pain and despair, and so on. It is usually silent, until Satan speaks, but a "horrid" silence. It is full of devils who when they do act, make terrible noises and do unspeakable things.
In the very long Poem "Paradise Lost" by Milton, I found several descriptions of Hell. In one section it is described as being:
“In the abhorred deep to utter woe;
"Where pain of Unextinguishable fire
Must exercise us without hope of end
The vassals of his anger, when the scourge
Inexorably, and the torturing hour” (line 87-91)"
In the wide womb of uncreated Night, 150
Devoid of sense and motion? and who knows,
"Our prison strong, this huge convex of Fire,
Outrageous to devour, immures us round
Ninefold, and gates of burning Adamant
Barred over us prohibit all egress."
"Rocks, caves, lakes, fens, bogs, dens, and shades of death,
A universe of death, which God by curse
Created evil, for evil only good,
Where all life dies, death lives, and nature breeds,
Perverse, all monstrous, all prodigious things,
Abominable, inutterable, and worse"
Satan had waged war against God and his angels. The angels defeated Satan and his devils. In the sections of the poem as listed, Satan has called together all of his devils in Hell. He desires to seek vengeance towards God for God creating the place where he has been cast. He is giving a speech to the devils. He describes hell in the process of his speech.
After the great battle in Heaven and the fall of the rebel angels.We are then shown(in Book I) a dark, bizzare and dismal place that torments the rebellious crew of Satan from its very sight.
Satan and his angels are described lying on a lake of fire in a place where flames cast no light, but only "darkness visible". Milton gives further descriptions of hell as 'A dungeon horrible', 'great furnace flamed' etc., And that it was to discover the sights of woe, regions of sorrow where peace or joy can never reinstate. It has an endless torture in it. The whirlwinds are of tempestous fire ever burning and serves as a contrast to the blissful Heaven.