regna pallida, dis invisa

One of the most exciting and vivid passages in Aeneid VIII comes at the climax of the story of Hercules and Cacus. Hercules has succeeded in tearing the roof from Cacus' cave, and Cacus responds by spewing out great clouds of smoke and fire.

at specus et Caci detecta apparuit ingens
regia, et umbrosae penitus patuere cavernae,
non secus ac si qua penitus vi terra dehiscens
infernas reseret sedes et regna recludat
pallida, dis invisa, superque immane barathrum
cernatur, trepident immisso lumine Manes.

But the cave and the giant palace of Cacus appear, uncovered, and the shadowy caverns deep within are revealed, just as if the earth, cracked wide open by some force, had unveiled the infernal dwellings deep below and laid bare those pale kingdoms, hateful to the gods, and as if the immense pit were seen from above, and the shades of the dead, trembling at the light let in.

ergo insperata deprensum luce repente
inclusumque cavo saxo atque insueta rudentem
desuper Alcides telis premit, omniaque arma
advocat et ramis vastisque molaribus instat.

And so Hercules presses down upon him with his weapons, suddenly trapped by the dreaded light, and imprisoned in his hollow rock and bellowing strange noises, and he summons up his arms and rains down upon him branches and great boulders.

ille autem, neque enim fuga iam super ulla pericli,
faucibus ingentem fumum (mirabile dictu)
evomit involvitque domum caligine caeca
prospectum eripiens oculis, glomeratque sub antro
fumiferam noctem commixtis igne tenebris.

But Cacus, since there was no longer any escape from the danger above, spews forth (incredible to speak of) great clouds of smoke from his jaws, and he wraps his home in blinding darkness, stealing the sight from his eyes, and he gathers the smoky night deep down in his cave, with the darkness mixed with fire.

(Aeneid VIII.241-255)

The simile in the opening lines draws on a similar passage from Homer's Iliad, where the fighting of the gods threatens to break open the earth:

And down below Hades, the lord of the dead, was terrified, and leapt screaming from his throne for fear that Poseidon the earthshaker would break open the earth above him, and the dwellings of the dead be revealed to mortals and immortals, the ghastly places of mould which the gods themselves hold in horror - such was the crash that arose as the gods joined in the conflict.

(Iliad XX.61ff)