Fluid dynamics is hard!
Kudos to Claude-Louis Navier and George Gabriel Stokes for deriving a nearly intractable equation relating fluid velocities, pressure gradients, gravity, and shear stresses! For the last three weeks I have sat in class as Dr. Spall listed step after step of mathemagical proof, starting with the laws of conservation of mass, momentum, and energy. The rigamoral they went through! Stress tensors! Divergence theorem! Taylor series expansion! Kronecker delta! It's seriously astounding. And they did it in the 19th century without the aid of computers, calculators, or even whiteboards!