By degrading the insulation. Electrical transformers are composed of a series of coiled electrical wires. The wires are sheathed in paper-based insulation, which prevents electricity from jumping across wires within the coil. Over the course of 20 to 60 years, depending on how hard the apparatus is made to work, the insulation in a well-maintained transformer degrades from 1,200 molecules thick to 200 molecules thick, at which point the coils should be replaced. Water accelerates that degradation process immensely. When the insulation fails, parts of the coil touch, causing a “turn-to-turn fault”—a form of short-circuit that creates a spark inside the transformer. The spark ignites the oil surrounding the coils, and the resulting explosion can be massive, as video from Monday night’s failure demonstrates.