Everyone seems to know it was Russia and is just casting out ideas to find a plausible motivation. I find that completely baffling. For opportunity, consider that while setting up the pipeline needs a weird high-pressure residence and special training, blowing it up takes a fishing boat, scuba gear and underwater explosives. The equipment and training is cheap and available. So, those with opportunity is almost anyon…
I just hope it wasn't the US. That would be a devastating breach of trust for Germany and would backfire badly if it came out. I'm rooting for the Danish anarchists. You basically just need underwater explosives and a boat. You hang the explosives on a long rope, throw it over board like an anchor, and drag it into the pipeline. When you hit the pipeline, you detonate the explosives.
That's genius. The detonator would not have to be very sophisticated to magnetically detect the steel in the pipeline.
"The gas pipes are made of 4.1 cm thick steel, which is protected by an 11 cm thick reinforced concrete shell"
https://yle.fi/uutiset/74-20000414 (in Finnish)