The US actually did that between the Mainland and Kamchatka peninsular.
There is a Russian military base on the peninsular .
The US wanted to tap into the communications but didn't know the exact location of the cable.
Secretly photographing the shores showed signs along the banks that read "Caution, Underwater Cable" or something like that.LOL
The US tapped the cable but the tap was eventually found.
Don't remember how long it was from tap to discovery but it was quite a while IIRC.