Dunderball is a sport of the Office Olympics, played by Toby Flenderson and Jim Halpert.
The game is played by bouncing a small ball on the wall and over the divider that separates Toby's and Kelly's work stations. A piece of blue tape on the wall separates the two sides. As explained by Toby in a deleted scene, players are permitted to hold the ball for one second, and the ball must strike the wall on the thrower's side of the tape.