Dante is reawakened by a loud peal of thunder. He looks around to try to figure out where he is and finds himself on the edge of the very pit of hell looking down into its chasm, which resounds with "infinite groans like gathered thunder," (4.9). He cannot see the bottom of the pit, which Virgil tells him they must journey down.
The "infinite groans" are the result of the suffering the souls in hell undergo as punishment for their sins. Their painful, inarticulate groans are in sharp contrast to Virgil and Dante's skilled speech.
Dante sees that Virgil is pale, and asks how he can be expected to go through hell, when even Virgil is frightened. But Virgil tells him that he is pale with pity for all the suffering souls in hell. Following behind Virgil, Dante enters the first circle of hell.
Virgil will later teach Dante not to feel pity for those who suffer punishment as part of God's divine justice. However, even he feels some pity for the damned souls. He may be Dante's poetic idol and model of virtue, but he's not perfect.
Dante hears not loud, suffering groans, but constant sighing. Virgil tells him that the souls in this first circle did not sin, but instead were either never baptized or lived before the time of Jesus (as Virgil himself did) and so were not practicing Christians. Virgil explains that those trapped here do not suffer, but live here in Limbo forever without any hope of progressing to heaven. Dante asks if anyone has ever made it from here to heaven, and Virgil tells of when Jesus came and rescued some chosen people from hell, including Adam, Noah, and David (all from the Old Testament).
As these souls demonstrate, even being virtuous is not enough to guarantee entrance to heaven. Limbo, as it is described here, offers Dante a way to exclude the pagan, non-Christian Greeks and Romans he admires from heaven while still respecting them and without subjecting them to the punishments found in the rest of hell.
Virgil identifies some of the souls for Dante: Homer and the Roman poets Horace, Ovid, and Lucan. These ancient poets come forward and speak with Virgil, who introduces them to Dante. They invite Dante into their group of esteemed poets.
These ancient writers are particularly admired by Dante for their poetic skill and excellence. Dante the author has them invite him (the character in his work) into their group as a way of asserting his own prominence and fame as a great poet.
Dante, now accompanied by these five ancient poets, comes upon a great castle, surrounded by walls. The group passes through the castle's gate and walks along a green meadow. Virgil points out to him a number of famous people from classical mythology and history: Electra, Hector, Aeneas, Caesar, and others. He also sees the medieval sultan Saladin (who fought against European crusaders), sitting alone.
Homer's Iliad tells the story of Hector, Virgil's Aeneid tells the story of Aeneas, and several Greek tragedies tell that of Electra. By including these characters in Limbo, Dante in some sense claims to subsume these great works in his own, all-encompassing epic. His poem contains elements of all the best classical works of literature, all wrapped into one (Christian) narrative.
Dante also sees the great luminaries of ancient philosophy: Socrates, Plato, Democritus, Heraclitus, Cicero, Seneca, and more. He sees Euclid, Ptolemy, and Galen (who studied and wrote about geometry, astronomy, and medicine, respectively). Dante says that he cannot list all that he saw, since "The wondrous truth outstrips my staggering pen," (4.147). Virgil and Dante leave the other poets behind, and move on further into hell, where there is no light whatsoever.
Dante claims that his pen cannot match the wonders he saw in hell, though elsewhere he is very confident in his poetic abilities. His naming of various great classical thinkers is a gesture of respect toward them, but also asserts his own superiority to them. They are, after all, trapped in Limbo, while he journeys on toward heaven.