Russian Spy Ship Yantar Loitering Near Trans-Atlantic Internet Cables (navalnews.com)

Or, far more likely, creating their own.

For the Celtic - Norse cable now is probably the ideal time to tap it as it does not appear to be operational yet. For other cables brining them down with some strategic use of an anchor and tapping before it is repaired would probably be very tricky to detect.

It seems to be happening for a pretty long time (at least the 1970s) https://en.wikipedia.org/wiki/Operation_Ivy_Bells

Very sophisticated work. Not impossible.

Isolate a single fiber, bend it slightly beyond its bend radius, measure the light that escapes. This will appear as a slight reduction in signal strength, but won't be conclusively identifiable as a tap. https://www.thefoa.org/tech/ref/appln/tap-fiber.html

You should read 'Blind Mans Bluff' - its about US submarines doing exactly that about 30 years ago.

https://www.amazon.com.au/Blind-Mans-Bluff-Submarine-Espiona...