A set of painted-on pairs of footprints you find at the Marine Corps Recruit Depot in San Diego, California ... and probably also in Parris Island, South Carolina. They're set up like like this:

 xx xx xx xx xx xx xx xx
 xx xx xx xx xx xx xx xx
 xx xx xx xx xx xx xx xx
 xx xx xx xx xx xx xx xx

After the drill instructors scream their lungs out at you to get out of their bus they make you stand on the yellow footprints. At this time you're in platoon formation. You've basically just breathed your last breath as a free man and you know it. It's going to be a long 12 weeks.

This moment and the footprints themselves are forever ingrained in the minds of those who have stood upon them.