4 ideas
In August 79 AD, young engineer Marcus Attilius Primus takes over the Aqua Augusta, the aqueduct supplying the Bay of Naples, after its previous engineer Exomnius vanishes. When the water stops flowing he leads a repair party to the fracture on Vesuvius's flank and finds the ground itself rising, the water turned sulphurous, and the conduit split.
A large engineered system that runs across the landscape records changes in that landscape before any human notices them. Falling pressure, sulphur in the water, dead fish in fed pools, and dry springs are routine maintenance faults on their own, but together they are measurements of magma rising. The engineer who reads the system's anomalies as data sees the catastrophe coming, while those who read them as omens or nuisances do not.
Pompeii had survived a major earthquake in 62 AD, and small tremors had since become background noise. Because each past warning had passed without disaster, residents treated escalating signs as more of the same, and the volcano was not even recognised as a volcano. Repeated survivable warnings train people to discount the one that is different in kind.
Once the people charged with maintaining a public system are paid by those who profit from it, faults go unreported and the system's warnings are suppressed. The corruption conceals exactly the information that could have saved the city.