Anti submarine blimps?
Like the ones you see in war films that they used to float up so that aeroplanes would catch on them?
This has to be the most outside the box DARPA concept ever - "I know. Lets try and catch submarines, by floating big balloons up in the air. Balloons tethered by cables so that if the submarine catches on the cable, it'll cause it to crash and burn. Its the one place that the submarines will never expect us to try to catch them: UP IN THE FRIKKIN SKY!"
Or maybe I missed the point of this somehow...