Russias's Sierra II-Class Has Titanium Twin-Pressure Hulls the U.S. Navy Can't Match
The Russian Navy's Sierra II-class submarines, also known as Project 945A Kondor class, remain among the most expensive and deep-diving boats ever built. As a follow-up to the Sierra I, the subs were specifically developed for search-and-destroy missions against U.S. nuclear-powered ballistic missile submarines.
Source: nationalinterest.org