The "most dangerous kind" is just regular warfare, i.e. physically disrupting infrastructure.
I'd put this on the same "level" as bombings and so forth.
it's way way more simpler than that. London's primary data store is in a number of buildings on the edge of Docklands/Isle of Dogs, on a road called Marsh Lane. Leading away from that, is limeharbour, and on this road there are at least 100 man hole covers, showing the logos of all the main transit companies (colt, etc). Here, let me show you: http://bit.ly/qvEXM
how hard, exactly, would it be to lift these covers and snap all the cables in sight?