[m4rtink]

Afaik unlike Nord stream the tunnel sections will be burried in a trench, under a lot of both rock ballast (so they don't float back up) and soil. Not impossible to breach of course, but requiring either a lot of some very serious military grade explosives (earthquake bombs, just underwater, somehow) or a very auspicious drilling rig.

[hagbard_c]

Depending on how much ballast is dumped on top of the segments a number of depth charges detonated on top of the thing might be enough, especially if detonated around the interface between two segments. If a depth charge is not deemed to be effective enough I'd imagine a container full of AnFo would suffice. I also assume the designers have thought about these possibilities, especially after the Nordstream 2 debacle and have taken care to make the design able to withstand such threats.