  We reprove the theorem of Feigin and Frenkel relating the center of the
critical level enveloping algebra of the Kac-Moody algebra for a semisimple Lie
algebra to opers (which are certain de Rham local systems with extra structure)
for the Langlands dual group. Our proof incorporates a construction of
Beilinson and Drinfeld relating the Feigin-Frenkel isomorphism to (more
classical) Langlands duality through the geometric Satake theorem.
