WARNING TRACK

A 'warning track' is the term for the part of the baseball field that is closest to the wall or fence and is typically made of dirt, instead of grass or artificial turf like most of the field. It runs parallel to the ballpark's wall and looks like a running track. 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.
The term "warning track", in common use, typically refers only to the outfield portion of the track, since infielders seldom encounter it, and even more rarely approach it at the high speeds of an outfielder trying to get under a deep fly ball. A batter who swings with all his might but has the ball caught at the warning track – only feet from scoring a home run — is said to have "warning track power."
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 Yankee Stadium, where an actual running track was built for the use of track and field events. This also helped outfielders know when they were approaching the wall, and soon every ballpark was using one.