As we have observed in previous chapters, the spaces underneath the city have always mapped ideological battles, from the class struggle in the bowels of the sewers to the redistribution of power through the construction of the underground railway. Although subterranean systems attempted to contain these agitations in the mid-nineteenth century, political discontent continued to foment and erupt on the surface of the city in more violent and explosive ways in the latter half of the century. Even the word ‘underground’ gained new currency, from a concrete, physical space to a more metaphorical representation of conspiratorial networks, extending its meaning to secret societies, crime syndicates and international political organisations. The OED maps this transformation from a concrete ‘region below the earth’ to a symbolic reference to all that is ‘hidden, concealed, secret’, tracing its political usage specifically to Sergei Kravchinsky's nihilist tract Underground Russia in 1883. The underground's link to subversive activity was pervasive, most notably in connection with the Parisian sewers that housed the city's criminals and revolutionaries, as well as in the underground railway, which offered a transgressive space of class and sexual encounters. In this chapter, however, I wish to locate the underground as a tactical site of political insurgency, as explosions in underground trains and bombs found in crypts began to signal a closer alliance between ‘underground’ and ‘terror’ in a material and a metaphorical way.