I’m a bit feared about the undersea communication cables.
so, the giant elephant in the room: this works just fine if it was done by US and/or allies. It is even a quite valid geopolitic strategic move - tough a bit crude, but hey this is war -, even risking the EU to discover it.
But then, the russians are the only other ones that actually know they didn't dit it. Hence, an automatic license to answer was granted. They know the other side won't answer to their answer, because of escalation to kinetic or worse, nuclear.
So, one possible, even the most feasible to be done without serious risk of escalation, would be to just go and sever some transatlantic cables. It's all the way international waters, it's accesible for underwater cover ops, it's not like messing with a nuclear reactor inside a state-nation, or cyber-trashing someones mainland infrastructure.
So, if the russians didn't did it, they have now some "credit" to spend. You can just wonder where and when they will do this.
Worst case would be someone damaging a lot of cables at once, e.g. near Highbridge, Skewjack, Bude (UK) or Bilbao (Portugal) - almost all Europe-US links end there [1], so a single sabotage submarine could place time-based explosives that cause a sudden catastrophic loss, large enough to overwhelm the capacity of cable runner ships.
In any case, a complete loss of communication between the US and Europe is highly unlikely. European traffic would simply be redirected among the bunch of SEA-ME-WE cables and hop from there over Malaysia and Japan to the US East Coast. A ton more latency and likely much less capacity along the route, but enough to survive.
[1] https://www.submarinenetworks.com/en/systems/asia-europe-afr...
[2] https://www.computerworld.com/article/3485615/capacity-upgra...
[3] https://www.networkworld.com/article/2235353/the-incredible-...