Engage in an impromptu discussion about the Hausdorff-Borel theorem (HB) in Lean with mathematician Mario Carneiro. Delve into the intricacies of formalizing this important mathematical concept within the Lean theorem prover, exploring its implementation, challenges, and implications for automated reasoning in mathematics. This 54-minute talk, presented at the Hausdorff Center for Mathematics, offers valuable insights for those interested in formal verification, theorem proving, and the intersection of mathematics and computer science.