The Fórnea of Porto de Mós is a natural depression (almost like a sinkhole)  in the shape of an amphitheater that is located in the Aire and Candeeiros Mountain Range Natural Park (Parque Natural das Serras de Aire e Candeeiros). It measures one kilometer in diameter and stands 250 meters high. Locals call it Fórnea because its shape is similar in shape to an oven (forno in Portuguese).

This geological phenomenon was caused by the erosion from rain and spring waters in the mountain range's limestone-rich soil. There are several springs in the valley, which are the main cause of the erosion that carved this natural amphitheater out of the mountain landscape.

The highest part of this natural wonder is located in the village Chão das Pias and it descends to another village called Alcaria. In the surrounding landscape, you can find low bushes like rosemary and hawthorn, as well as trees including fig, laurel, and strawberry. 

There is a trail of about one kilometer that begins in Alcaria and leads through the valley all the way to the basin of the Fórnea, where you can see several waterfalls and also a cave 500-meter-long cave called Cova da Velha (Hole of the Old Hag) where sometimes you can also see a spring. There is another trail all around the ridge of the Cornea (also about one kilometer in length). If you are feeling really adventurous, you can link the two trails.