The murder of Kim Wall, known in Denmark as Ubådssagen, took place on 10 August 2017, after Swedish freelance journalist Kim Wall boarded the midget submarine UC3 Nautilus, in Køge Bugt, Denmark, with the intent of interviewing its owner, Danish entrepreneur Peter Madsen.

UC3 Nautilus was a privately built Danish midget submarine. It was built over a three-year period by Peter Madsen and a group of volunteers, and cost approximately US$200,000 to build. The submarine was Madsen's third submarine design.