The Undertaker had never lost at WrestleMania, compiling a truly legendary 21-0 record. It was widely considered fact that "The Streak" would never be broken.
Brock Lesnar changed all that at WrestleMania 30. In perhaps the biggest upset in professional wrestling history, Lesnar defeated the Undertaker, leaving every witness absolutely speechless. The expression on the man's face above sums it up well.
Here are some more reaction shots.