On 20 September 1918, Austro-Hungarian U-47 torpedoed and sank the French submarine Circé. Both submarines were on the surface. The encounter therefore does not count as one submerged submarine sinking another underwater. That distinction matters when tallying submarine-on-submarine kills.
Notable Point · 2:12:19-2:12:40
U-47 sinks Circé while both submarines are surfaced
Austro-Hungarian U-47 torpedoes the French submarine Circé while both boats are operating on the surface.