Ghost Subs?

We've all read about ghost guns. But what about ghost subs? It turns out the newer classes of Russian submarines such as the diesel-electric Krasnodar are extremely quiet and are like noiseless spots in the ocean. The Wall Street Journal examines them as well as a deep-diving compatriot (Yasen class) in the video below. I wonder how much of the stealthiness of the Krasnodar was stolen technology courtesy of spys and/or traitors like the Walkers.

The Australian goes into much more detail in this story about the US Navy's search for the Krasnodar in the Mediterranean Sea.