A dramatic, atmospheric illustration in the style of classic WWII naval art. The central image is a dark, menacing German U-boat partially submerged in stormy, choppy ocean waters. Waves are crashing over its conning tower. Subtle lighting from a stormy sky (dark greys, deep blues, hints of muted yellow/orange on the horizon) illuminates the scene. The overall mood is tense, isolated, and dangerous. The composition should be dynamic, with the U-boat angled slightly, implying movement and threat. Focus on realistic textures for the metal of the submarine and the churning water. The aesthetic should be that of a professional, impactful book cover for a historical thriller. Aspect ratio: 1024x1024. No text.