Greekfire or Wildfire for Game of Throne fans. Cersei blows up the Great Sept of Baelor clip, https://www.youtube.com/watch?v=q6oO4p9ghfo, by filling the undergrounds with the green wildfire.

But, using the sewers has been mentioned in the Protocols.

"We have a maneuver of such appalling terror that the very stoutest hearts will quail--the undergrounds, metropolitans, those subterranean corridors which, before the time comes, will be driven under all the capitals to be blown sky high into the air." (The Protocols of the Elders of Zion)