Attilius is a young Roman aqueduct engineer dispatched from Rome to the Bay of Naples to maintain the Aqua Augusta. Recently widowed, he throws himself into his work, using engineering mathematics as a reliable framework in an unpredictable world. He faces resentment from his older crew members but remains determined to fulfill his familial duty as the latest in a long line of civil engineers.

Ampliatus is a former enslaved man who has amassed a vast fortune through property speculation in Pompeii. He wields immense influence over local magistrates and maintains a cynical view of society, actively constructing a public image of legitimate success while operating criminal schemes behind the scenes. He punishes perceived slights violently, feeding disobedient workers to his moray eels.

Corelia is the teenage daughter of Ampliatus. She resents her father's corruption, actively defining her morals in opposition to his ruthless methods. She demonstrates compassion for the enslaved workers at the Villa Hortensia and attempts to assert her own agency in a patriarchal society that routinely sidelines women.

Pliny is a Roman admiral, military veteran, and author of scientific encyclopedias stationed at Misenum. He values empirical observation over political maneuvering. His genuine fascination with the natural world isolates him from the decadence of the ruling elite, leading him to utilize his naval resources to document geological anomalies.

Corax is the senior overseer on the Aqua Augusta engineering team. He harbors deep resentment toward Attilius due to the younger man's rapid promotion and strict managerial style. He frequently undermines repair efforts and attempts to turn the working-class crew against their new supervisor.

Popidius is an elected magistrate in Pompeii. He previously owned Ampliatus but now finds himself deeply indebted to the wealthy former slave. He maintains an air of aristocratic superiority while privately struggling with his compromised financial position and a forced betrothal to a teenager.

Exomnius is a seasoned aqueduct engineer from Sicily who previously managed the Aqua Augusta. His sudden disappearance leaves behind disorganized records and unpaid rent at a local brothel, sparking rumors about his secretive financial dealings.

Celsinus is the son of Ampliatus. He occupies a privileged position within his wealthy family but displays protective instincts toward his sister when household tensions flare.

Sabina is the deceased wife of Attilius. She died tragically during childbirth at the age of twenty-two. Her passing profoundly traumatized Attilius, shaping his emotional detachment and obsessive work ethic.

Atia is an enslaved woman working in the kitchens of Villa Hortensia. She suffers severe physical abuse after her son is accused of poisoning valuable fish, prompting her to seek justice with Corelia's help.

Hipponax is an enslaved worker tasked with feeding red mullet at the Villa Hortensia. When the fish die from sulfur contamination in the water supply, he is falsely blamed and condemned to execution as a display of aristocratic power.

Gaius is the nephew and heir of the famous admiral Pliny. He assists his uncle with research, displaying a cautious temperament when confronted with dangerous natural phenomena in the bay.

Brebix is a former gladiator who purchased his freedom and now works as a senior laborer for Ampliatus. He possesses imposing physical strength and harbors deep superstitions about underground giants rumbling beneath the mountain.

Taedia Secunda is a sixty-year-old woman and the former wife of Popidius. After Ampliatus orchestrated her divorce, he forced her into sexual servitude as an ongoing form of revenge against his former masters.

Africanus owns a local brothel in Pompeii. He runs a disordered establishment and attempts to hide the extent of his tenants' secretive activities from investigators.

Zmyrina is an aging sex worker employed at Africanus's brothel. She possesses knowledge of Exomnius's unfulfilled promises and hidden financial dealings before his sudden disappearance.

Tiro is a blind water-worker stationed at the Pompeii reservoir. He endures bullying from the other laborers while possessing valuable knowledge of local seismic activity and the habits of the previous aquarius.

Musa is a laborer on the Aqua Augusta engineering team. He complains about the unequal division of work and remains skeptical of the new supervisor while aligning himself with the antagonistic Corax.

Polites is an enslaved member of the aqueduct maintenance team. He handles logistical tasks like organizing equipment wagons and faces mistreatment from the free members of the work crew.

Pomponianus is a military comrade of Pliny. He owns a coastal villa and favors traditional religious explanations over scientific observation when confronting natural disturbances.