Natural gas exports to Europe account for a major proportion of Russian export income.
Russia has quarreled continually with Ukraine over the price of gas, leading to a showdown in the winter of 2009 during which supplies to much of Europe were cut off. They have recently almost doubled the price of natural gas to Ukraine, effectively declaring an economic war on a country they hypocritically call a “Slavic Brother.”
Russia exports its natural gas to Europe through Ukraine and the Nord Stream pipeline under the Baltic Sea. They built Nord Stream to end their dependence on Ukraine.
Nord Stream is 1220 kilometers long, terminating in Germany. The Baltic Sea is shallow. For most of its length, the pipeline is at depths a Scuba diver could reach. Or, perhaps, a Navy Seal. Perhaps a Navy Seal sapper. Perhaps without uniforms or insignia, like the insurgents who took over Crimea.
I would propose that declaring an economic war might not be such a good idea for Russia. People in glass houses shouldn’t throw stones, and they have been lobbing a lot. Some might come back.