It always strikes me that terrorists lack imagination. Running amok with a knife or blowing yourself up certainly creates headlines but if you really wanted to cause mayhem a little online research would give you all the information you needed to poison the water supply to an entire community or even city. For example the aqueduct from Thirlmere to Manchester and the massive reservoir at Heaton Park are easy to find and easy to access with a little effort. Maybe terrorists think water just comes out of taps and don't think about its origins. Above my house there's a feeder reservoir for the town, which is surrounded by 20' electric fences with two CCTV cameras at each corner pointing both ways so United Utilities must be aware there's a risk.