David Hilbert posed 23 unsolved mathematical problems in 1900 that became known as Hilbert's problems.
Notes on verification
Well-documented historical fact confirmed by multiple independent, authoritative sources including Wikipedia, MathWorld, and Simons Foundation. Minor nuance that only 10 of 23 were presented orally does not undermine the claim's substance. [tier=bronze indep_score=0.3 clusters=2 claim_tier=everyday]