How does Grendel's lair remind you of hell in Beowulf?

Beowulf is an epic full of imaginative detail, and the poem's description of Grendel's lair is no exception. The image evoked can certainly be considered hellish upon close examination.
It might be helpful to first set up Grendel's lair in contrast to Heorot, Hrothgar's mead-hall. This place of merriment offends Grendel to the point that he attacks it and its inhabitants nightly, and considering that Grendel is said to be a descendent of Cain, outcast from God's favor, this makes perfect sense. Heorot is described in the prologue as a place not only majestic, but also heavenly. Hrothgar swears to build a hall for his people that "would hold his mighty / Band and reach higher toward Heaven than anything / That had ever been known to the sons of men" (lines 68–70). It is referred to as the "most beautiful of dwellings" (77). As we meet Grendel in the first stanza of the poem after the prologue, he and his lair are presented as the absolute inverse of Hrothgar's people and Heorot:

A powerful monster, living downIn the darkness, growled in pain, impatientAs day after day the music rangLoud in that hall, the harp's rejoicingCall and the poet's clear songs, sungOf the ancient beginnings of us all, recallingThe Almighty making the earth, shapingThese beautiful plains marked off by oceans,Then proudly setting the sun and moonTo glow across the land and light it;The corners of the earth were made lovely with treesAnd leaves, made quick with life, with eachOf the nations who now move on its face. And thenAs now warriors sang of their pleasure:So Hrothgar's men lived happy in his hallTill the monster stirred, that demon, that fiend,Grendel, who haunted the moors, the wildMarshes, and made his home in a hellNot hell but earth. (1–19)

The poem makes it clear that Grendel represents complete opposition to everything good and holy, such that his lair is described as a kind of hell on Earth. His home is continually described throughout the poem as a place of darkness, and he's known to drag the bodies of the men he kills back to the lair; he punishes them for their joy and mirth by taking them to a place that is more akin to hell than the heavenly mead-hall their king built for them.