David Hilbert proposed a famous list of 23 unsolved problems in mathematics in 1900, known as Hilbert's problems.
Notes on verification
Well-documented historical fact confirmed by Wikipedia, MathWorld, and Simons Foundation. Minor nuance that only 10 of 23 were orally presented does not undermine the core claim. [tier=bronze indep_score=0.3 clusters=2 claim_tier=everyday]