A properly designed grass football field does not lie perfectly flat. The surface of the field often arcs a foot or more from the sidelines to the midfield to allow excess water to drain off. Without a crown, as the raised area is called, a grass football field can become a soupy, dangerous mess.
https://www.sportsrec.com/417272-what-is-the-crown-of-a-football-field.html
Feet / legs won't disappear. Try it. Basketball courts are flat.