This is one of those ones where my instinct is "no": not only would you have to not cause an interruption or reflection that the break detection TDR systems could see, and crack any encryption, and sample what you want from the Tbps, all from a small box under the sea, but also you have to somehow get that data out and back to base, again from under the (mostly radio-opaque) sea and halfway around the world, all without even a whisper of a clue to the tappees.
Then I remember how far ahead the likes of the NSA and NRO are compared to what we're familiar with, and become rather less sure. The Orion satellites have 100m radio dishes, and were first launched in the 90s. Two Hubble-like telescopes were so old hat that they were donated to NASA in 2012. Considering that the NRO is so secrecy-oriented that its very existence was classified until 1992 (it went 11 years completely undetected, and leaked via a New York Times article in 1971 and an accidental entry in a budget report in 1973) and no mission since 1972 is declassified, this says a lot about how much further on they are.
Then again, if unattended taps were installed on cables, you'd also expect them to occasionally be found when lifting cables for repair. And they'd be so advanced that it might be worth lifting an entire cable to check for and acquire such a tap. Which means the tappers would think twice about putting one in, if they could then lose it.
> if unattended taps were installed on cables, you'd also expect them to occasionally be found when lifting cables for repair
<conspiracy theory> An advanced enough attacker would build their cable taps in such a way that they automatically dropped off when they detected the cable being lifted - and would probably result in suspected but not provable "damage caused by human activity" that has broken through the cable armouring and exposed the fibre bundle inside. Now I'm wondering if the Svalbard cable damage was a software bug in the cable tap device.
even the metadata would be valuable though, so you wouldn't need to crack the encryption, and you don't have to have it be real-time, so you can just process and save the relevant data and pick it up later, so my instinct is that it's possible there's something there, but it would be really difficult, and we might hear about it in 50 years, just like we learned about Bletchley Park.
Then I remember how far ahead the likes of the NSA and NRO are compared to what we're familiar with, and become rather less sure. The Orion satellites have 100m radio dishes, and were first launched in the 90s. Two Hubble-like telescopes were so old hat that they were donated to NASA in 2012. Considering that the NRO is so secrecy-oriented that its very existence was classified until 1992 (it went 11 years completely undetected, and leaked via a New York Times article in 1971 and an accidental entry in a budget report in 1973) and no mission since 1972 is declassified, this says a lot about how much further on they are.
Then again, if unattended taps were installed on cables, you'd also expect them to occasionally be found when lifting cables for repair. And they'd be so advanced that it might be worth lifting an entire cable to check for and acquire such a tap. Which means the tappers would think twice about putting one in, if they could then lose it.