Operation Ivy Bells→

Wikipedia:

In October 1971 the United States sent the purpose-modified submarine USS Halibut (SSGN-587) deep into the Sea of Okhotsk [(Soviet territorial waters)]. Divers working from the Halibut found the cable in 400 ft (120 m) of water and installed a 20 ft (6.1 m) long device, which wrapped around the cable without piercing its casing and recorded all communications made over it. The large recording device was designed to detach if the cable was raised for repair.

Even cooler:

Eventually, more taps were installed on Soviet lines in other parts of the world, with more advanced instruments built by AT&T’s Bell Laboratories that were nuclear-powered and could store a year’s worth of data.

Gotta love the cold war. “Oh, yeah, I built a nuclear-powered wiretap. No big deal.”