Blowing it up is relatively easy.

Use depth charges, or even a homing torpedo (it must look like a submarine resting on the bottom),

I don't know much about this technology either, and there seems to be very little information available about it. I've heard
that even though the line was not in use, it was filled with pressurized gas. A breach would cause it to leak, but for how long?
I've also heard salt water can damage the interior of the pipe, but I don't know what the repair and maintenance procedures are,
surely there are ways to repair this in the event of an accident or natural disaster. Does the pipe have a sealing mechanism that can isolate a breach?
I don't know and nobody's talking.

Undersea cables break occasionally, and there are means to fix them and bring them back on line quickly, I presume its much the same for pipelines.