A warning track is the term for the part of the baseball field

Baseball field

A baseball field, also called a ball field or a baseball diamond, is the field upon which the game of baseball is played. The terms "baseball field" and "ball field" are also often used as synonyms for ballpark.-Specifications:...

 that is closest to the wall or fence and is typically made of dirt, instead of grass

Grass

Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...

 or artificial turf

Artificial turf

Artificial turf is a surface manufactured from synthetic fibers made to look like natural grass. It is most often used in arenas for sports that were originally or are normally played on grass. However, it is now being used on residential lawns and commercial applications as well...

 like most of the field. It runs parallel

Parallel (geometry)

Parallelism is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The assumed existence and properties of parallel lines are the basis of Euclid's parallel postulate. Two lines in a plane that do not...

 to the ballpark's wall and looks like a running track

Track and field

Track and field is a sport comprising various competitive athletic contests based around the activities of running, jumping and throwing. The name of the sport derives from the venue for the competitions: a stadium which features an oval running track surrounding a grassy area...

. The change of terrain from grass to dirt serves as a "warning" for fielders trying to make a deep catch that they are running out of room, since it is often difficult for the fielder to keep his eye on a fly ball while keeping track of his position relative to the wall.

Despite the warning track's presence, it is common to see outfielders crash into the wall to make a catch, due either to a desire to field the play regardless of the outcome or because they fail to register the warning.

The term "warning track" comes from Old Yankee Stadium, where an actual running track was built for the use of track and field

Track and field

Track and field is a sport comprising various competitive athletic contests based around the activities of running, jumping and throwing. The name of the sport derives from the venue for the competitions: a stadium which features an oval running track surrounding a grassy area...

events. This also helped outfielders know when they were approaching the wall, and soon every ballpark was using one.