The Cave of the Circle, also known as the Magic Circle Cave, is a location in Final Fantasy III accessed from the back room of Doga's Manor. Doga accompanies the party to the end of the cave in order to make the Nautilus submersible. The party must stay in Mini status throughout the cave.
In order to obtain the Eureka Key, Doga orders the Warriors of Light to accompany him through the Cave of the Circle. At the bottom, he warps himself to another dimension, and the Warriors back to his manor, after casting a spell that allows the Nautilus to submerse itself. Then, he asks the Warriors to wake Unei from her sleep by obtaining Noah's Lute at the Temple of Time.
The background theme that plays inside the Cave of the Circle is called "Doga and Unei".