Dozens of fibre-optic cables span the globe and Nato also has dedicated military cables on the ocean floor.
The Yantar is capable of tampering with them, says Igor Sutyagin, a London-based expert on the Russian military. But there is no evidence that it has done so.
"It's difficult to tap into optical fibres - it's just light inside, not electrical data," he told the BBC. "It would be easier just to cut the cable."
Mr Sutyagin noted that in the 1970s, during the Cold War, the US Navy had lost control of a Sosus undersea listening post for tracking submarines in the Atlantic. Sosus stands for "Sound Surveillance System".
The US military concluded that a Soviet submarine had cut the cables.
Mr Sutyagin, of the Royal United Services Institute, says the Yantar belongs to Russia's Main Directorate of Underwater Research (GUGI in Russian), part of the defence ministry.
The 108m-long (354ft) vessel has a crew of 60 and went into service in 2015. It was built in the Baltic port of Kaliningrad, the first in a series called Project 22010. A second, called Almaz, will soon be on its way to the navy.