According to some experts it may have been done using mines and divers deployed from a sub. (The use of mines is apparently somewhat medieval in this context, as there are better methods for an experienced team.) Given the density of sensor networks in the area, you'd probably need a really quiet sub, something like the German Type 212 / 212A, including export models and license builds. I guess, the Russians have something similar. (Not an expert myself by any means.) So there is quite a number of candidates in the vicinity.
P.S.: With regard to these sensor networks, I guess, the "dark" surface vessels weren't big news for the military in the area.