From page 25 of Connected:
A group of physicists who usually study waves on the surface of liquids were sufficiently intrigued that they decided to study a collection of filmed examples of La Ola in enormous soccer stadiums; they noticed that these waves usually rolled in a clockwise direction and consistently moved at a speed of “twenty seats per second”.*
Damn! I have participated numerous times in such waves and never, ever thought of that!
[*] – “Mexican waves in an excitable medium“, Nature 419, 131-132 (2002).