Edward Snowden revealed a long time ago that the NSA also taps the undersea communications cables for mass surveillance (which obviously includes US citizens).
That was hardly a Snowden leak. The Russians have had one of the Ivy Bells listening taps on public display since 1999. The book Blind Man's Bluff details the taps and was published in 1998.
These days, tapping them isn't likely to be particularly useful. Anything sensitive transiting them is gonna be encrypted. Ivy Bells was possible because the Soviets thought it would be impossible to tap the cables - they were within Soviet waters and the tech used to finally do it was quite clever - so they didn't encrypt anything.
