What are loops?

In "The mazes" card, "Loops":

• None: every maze a tree (classic).
• Some: a third of the dead ends become loops.
• Many: most dead ends become loops. Loops let the solver walk in circles, which makes a maze less predictable. Yet each maze keeps exactly one way through: loops are only made where they cannot open a second way to the finish, and the check proves it for every maze. How to play explains loops to the reader.