Oscar-class (I, II, III) SSGN subs, are double-hulled boats. You probably saw the massive pressure hull, then the inner hulls. In between, they normally put ballast tanks and batteries etc.
It'd be a repugnant job draining and inspecting the
Kursk.
btw, anyone read Sherry Sontag & Christopher Drew's
"Blind Man's Bluff"?