The large elliptical-shaped Flegethron (or Alexandros) crater occupies the southeastern part of the area, intersecting with Megalos Polyvotis.

Towards the southwest, the crater collapsed and filled with its own debris. The history of this hydrothermal crater goes back to 1971, when the first recorded hydrothermal eruption took place within the western part of Lakki and as a result, gave birth to Polyvotis and Flegethron. The activity continued during 1973, accompanied by steam effusion, loud explosions and lithic ejecta from the Lofos Rhyodacitic dome, without any signs of magmatic activity. The floor of the southern part is covered with lacustrine sediments.