Terceira Island, Azores, PT
These two communicating caves of recent volcanic formation have an interesting characteristic: they are in perfect balance with each other. Both are approximately 70 meters deep and connected through a narrow corridor. The first is a basalt chimney covered in vegetation and open to the sky. The second has its vault confined in the trachyte mountain. The chimney has a reading of Å 15,500. The twin cave, on the other hand, has the typical behavior of underground caves: the reading presents Å 0, but by inverting the reading scale, we are given a negative value of Å -15,500.
Therefore, at the same altitude (the two red dots in the diagram) we have a typically terrestrial environment and a typically underground one, in perfect balance.
