Macaron foot
pied
The pied — French for "foot" — is the porous ring at the base of a macaron that forms in the first minutes of baking. By then the surface of the shell has dried and no longer lets steam through, so the expanding air escapes sideways and lifts the edges.
An even, modest foot is the sign of correct technique, and its appearance reads the faults. No foot at all means the shells were not left to skin over, or the batter was over-mixed. A ragged foot pushing out to one side points to overheating or uneven heat. An excessively tall, hollow foot points to too high a temperature at the start of the bake.
A common separate problem is a foot that rises and then sinks along with the dome. That usually means the bake was too short: the internal structure had not set, and the shell collapsed as it cooled.