Relevant bit is called a branching unit, eg Huawei BU1650 which is basically a switch/splitter that works usually with an amp/repeater to split signals off to drop at a landing station. They can be attached to the cable offshore, otherwise the cables are built through the landing stations in sections with the PFE and SLTs (Submarine Laser Terminals) powering and lighting each section. Adding one could get tricky given the power and optical signals are closely watched and cuts detectable very quickly. Then fire a TDR test from landing stations both ends of the cable to measure how far along the cable the cut is and send the co-ordinates to a cable ship to start looking for the cut.
They use either an ROV to visually follow and inspect the cable so may send a picture back of something unexpected. Alternatively the cable ship hooks up the section where the damage is, or cuts either side and splices a new cable section in. If there's an unexpected cable hanging off it, that would easily be noticed. Old copper cables could allegedly be clamped and monitored with the clamps detaching if they were disturbed.
For fibre, the idea of a parasitic clamp that could monitor a fibre without cutting it is less plausible. You could possibly do it via microbends but they'd require exposing the fibre which is in the middle of the cable surrounded by the cladding, armor wire and chunky copper power core. Which on a long distance cable like a transatlantic or transpacific one would probably be running 30-50kV DC. Exposing that to sea water could allow rapid detachment via the resulting short and steam explosion.
If you're not running the tap cable back to your own landing station, you'd then have to manage the data. That's not hundreds of megabits, it's usually n x 10/40/100Gbps. So an off-shore data logger would need to be a combination of DWDM mux, DPI system and storage, which would need it's own power and communications. Perhaps this is where the Google barges ended up? ULF isn't exactly practical if you're trying to send data transmitted originally at Ghz via ULF at a a few hundred hertz, or a few bits per second. That could involve rather a lot of buffering.
But if all those challenges are overcome, then it would be possible. A while ago I did look into the practicality of creating an off-shore PoP with a mux and router. Operating Juniper or Cisco at those depths would be tricky, not to mention voiding their warranties. They don't include deep-sea divers in their SmartNet contracts either.
Biting the hand that feeds IT
