This article is from the Drum corps FAQ, by Cathy Doser firstname.lastname@example.org with numerous contributions by others.
It is the area between the 35 yard lines on the front side line
ten feet deep (outside the playing field, toward the audience). Basically,
this area is an extension of the field. Any piece of equipment can be
brought into or out of the pit area to or from the "normal" playing field.
Corps typically use this area to place stationary percussion
instruments, such as chimes, keyboard instruments, tympani, gongs,
etc. It is also used sometimes to store color guard equipment. Some
drill writers have used the pit area for performers to march through.