In Book I, Hell is described as a fiery prison. Satan is thrown into Hell, in "Adamantine" (inflexible) chains and "penal" (punishing) fire, chained on a burning lake. As Satan views his environment in Hell, he sees Hell as:
A Dungeon horrible, on all sides round
As one great Furnace flam'd, yet from those flames
No light, but rather darkness visible
Serv'd only to discover sights of woe,
Regions of sorrow, doleful shades, where peace
And rest can never dwell, hope never comes (61-66).
The lake is in flames. Even though there are flames, they provide no light. Hell is in "utter darkness"; the only light serves to illuminate suffering. Hell is as far away from "God and light of Heav'n" as three times the distance between the Earth (or center of the Earth) and the outermost atmosphere (lines 72-74).
The so-called "chains" could be metaphoric. In other words, they may not be actual chains; the chains might refer to Satan and his cohorts being imprisoned/confined to Hell. This is possibly a metaphor, rather than a literal set of chains, because Satan later famously explains that it is "Better to reign in Hell, then to serve in Heav'n." (263) In either case, although Hell is a miserable place of eternal suffering, Satan rationalizes that it is better to suffer and be "free" than it is to serve God and live in paradise.
Satan rationalizes that he can make Hell bearable by force of will. This is one of the reasons Satan is portrayed as an odd kind of fallen hero as well as a delusional villain:
The mind is its own place, and in it self
Can make a Heav'n of Hell, a Hell of Heav'n. (254-55)
In his determination and pride, Satan determines to make Hell into something livable or even sublime.