Mathematics
The Classification of Finite Simple Groups
Quick fact
The classification of finite simple groups, completed in 2004, spans over 10,000 pages across hundreds of journal articles. Its proof is so lengthy that a full, error-free verification is still an ongoing challenge.
Why this is interesting
Imagine trying to list every type of LEGO brick that can build any structure, and the answer turns out to be a list so long and complex that it fills thousands of pages. That's exactly what mathematicians did for the building blocks of symmetry—but they called it the 'enormous theorem.'