As he describes Professor Lidenbrock’s house, Verne uses allusion and foreshadowing to subtly suggest the secretive and scholarly activities taking place inside. As Axel first introduces the house to the reader, he says:
The roof inclined to one side, in the position a German student belonging to the Tugendbund wears his cap.
The allusion to the house’s tilted roof—which sits “in the position a German student belonging to the Tugendbund wears his cap”—refers to a specific historical group of people from the early 1800s. The Tugendbund was a German secret society known primarily for two things: their pursuit of Prussian moral and intellectual ideals and their opposition to the rule of Emperor Napoleon. By likening Lidenbrock’s roof to the cap worn by Tugenbund members, the passage subtly suggests that the house is engaged in scholarly—and perhaps secretive—pursuits. This allusion gives the house a personality that mirrors its occupants and also hints at the intellectual and covert adventures that are about to unfold inside.
In this way, the allusion in the description of the house serves as foreshadowing, as it hints at subterfuge and the pursuit of hidden knowledge. This foreshadowing technique primes readers to expect that secrets are soon to be revealed, or that secretive deeds will be taking place within the walls of the professor's house.
As the men depart for the countryside, Fridrikson wishes them well by alluding to Virgil’s famous line, “Et quacumque viam dederit fortuna sequamur.” This does two things: it demonstrates his refinement and intelligence, and it also foreshadows the approaching disturbances and trials. As they leave, Axel tries to say a heartfelt goodbye:
I launched out a cordial farewell in the best Latin I could find; then we jumped into our saddles, and M. Fridrikson repeated with his last adieux that line of Virgil which seemed to have been made for uncertain travellers on the road, like us: Et quacumque viam dederit fortuna sequamur.
This allusion to the Roman poet Virgil's line from the Aeneid—usually translated as something like “And whatever route fortune gives, we will follow”—is a poignant moment. Fridrikson speaks Latin here partially because it is the only language in which he and Axel can communicate directly. He could have spoken Icelandic or Danish, but instead chooses to address the narrator in a way that includes him.
The phrase itself is from Virgil’s epic poem The Aeneid, which brings the narrative of the poem to mind as the men set off on their journey. In The Aeneid, the protagonist Aeneas embarks on a dangerous journey. After a long and winding path, Aeneas also descends into the underworld and eventually ends up in roughly the same geographic region as Axel does. His travels are filled with unforeseen challenges and adventures, and he has to keep his wits about him in order to avoid disaster. In using this particular line of Virgil’s, Fridrikson not only wishes the travelers well, but also acknowledges the potentially hazardous nature of their expedition. Like Aeneas, there's no guarantee they'll return after trespassing into unknown realms.
At the beginning of the final stretch of the men's above-ground journey, Verne employs some highly ominous foreshadowing to hint at the perilous and unpredictable Icelandic terrain the party will face. Long before they get close to penetrating the Earth’s surface, Axel wonders:
What, then, must those convulsed regions be, overturned by eruptions, themselves the offspring of volcanic explosions and subterranean commotions? We were doomed to know that in good time. But on consulting Olsen’s map I saw they would be avoided by keeping along the winding coast.
This description of these “convulsed regions” and areas “overturned by eruptions” provides a foreboding glimpse into the increasingly dangerous and unpredictable lands that await. It’s important to note here that the party has only just left “civilization” to try and reach the passage underground. The fact that Axel is already musing over what truly "unpopulated" areas might be like—especially if their current "cultivated" location is already so desolate and wild—is a chilling omen. It implies that the challenges ahead could be far more extreme than he anticipated. The mention of volcanic explosions and subterranean disturbances makes the reader aware of the reality of the earthquakes. It reminds them that the landscape is dramatic and rugged because of the seismic activities that have shaped Iceland. This doesn’t seem to bode well for an imminent descent into a live volcano.
Furthermore, Axel’s statement that "[they] were doomed to know that in good time" directly foreshadows some alarming and imminent encounters. He’s explicitly telling the reader, from his future perspective as narrator, that all is not going to remain well for long. This foreshadowing contributes to the already building suspense and anticipation for his adventure. Moreover, the narrator’s comment about the prehistoric “overturning” of the region by previous eruptions subtly hints at the climactic moment of volcanic explosion to come at the novel's climax.