How has Vergil created a grim and oppressive atmosphere in these passages?
In the first extract, Vergil describes the entrance to the underworld. He uses a simile comparing the journey to walking at night in a forest, "quale per incertam ... iter in silvis," to create gloomy imagery to portray the underworld as grim. To strengthen this, he lists a series of ills "Luctus...Curae...Morbi...Senectus" to overwhelm the reader with the threatening and thus oppressive nature of the underworld. Vergil supplements the oppressive effect through word choice describing loneliness ("sola ... vacuas ... inania"), showing that Aeneas and the Sibyl don't have anyone to help them, which creates an unsettling mood.
In the second extract, Vergil describes the scenery inside the underworld. He first describes the gargantuan scale of Charybdis ("vastaque voragine") and its strength in disturbing the landscape ("omnem eructat harenam") to show the unnatural power of the underworld, this makes it seem dangerous and oppressive. Then Vergil characterises the underworld as dirty/filthy with a lexical chain "caeno vastaque voragine ... terribili squalore ... inculta ... sordidus," showing it to be repulsive and grim.
In the third extract, Vergil describes groups of souls that inhabit the underworld. He includes a vignette about Minos judging the lives of the souls ("vitasque et crimina discit"), to show that the underworld knows everybody's secrets, making it seem invasive and oppressive. Vergil's authorial comment "quam vellent ... preferre labores" shows the souls would rather be alive and suffering than being in the underworld, implying it to be very grim and unpleasant. Finally, Vergil uses a series of verbs referring to restriction "obstat... alligat ... coercet ... non...relinquunt" to vividly portray how the underworld oppresses its inhabitants.