Mathematical proofs getting harder to verify

New Scientist: "However, this tiny uncertainty did not disappear with time. 'After four years they came back to me and said they were still 99% sure that the proof was correct, but this time they said were they exhausted from checking the proof.'

As a result, the journal then took the unusual step of publishing the paper without complete certification from the referees (Annals of Mathematics Vol. 162, p. 1063-1183, 2005)."