Terrorists! We call them Travelers in Scotland..

http://www.bbc.co.uk/news/uk-scotland-glasgow-west-25432545

Substations within the Grid aren't the weakest link, its the nuts and bolts on the legs of the HV Transmission pylons on the interconnects between the grid nodes. These are virtually un-defensible against a synchronized terrorist attack.

Dig up a few (around 30 Fibre Optic cable fat pipes on the East and west coast on the USA) and the Internet could also be taken down across the world

BTW Travelers in the UK, armed with a JCB digger has also been able to disable the most important RAF Air force base in the UK, but the News Media haven't been able to tell anyone.