Someone with a high degree of Electrical control could use some form of Electrolysis: http://en.wikipedia.org/wiki/Electrolysis
Basically, and with less science, you split water (H20) into Hydrogen (H2) and Oxygen (O), which is an incredibly volatile mixture. If this process goes on for a while, filling up the entire sewer/water system, the water levels would drop considerably (leading to an investigation) with the system flooding with Hydrogen gas.
All it takes down there, once it's filled up, is for a hero to light up a spark - and there's a sudden pop of a hydrogen explosion.
Not entirely realistic, scientifically, but hey - trying to outrun a hydrogen explosion shockwave sounds fun, doesn't it?