Gap Alzola is a natural cavity developed reef limestones and interbedded carbonaceous layers of lignite. artificial mine galleries were drilled last century to exploit the lignite . Currently the mine cave and form a network of tunnels that is crossed by an underground river. The cavity has a variety of speleothems of iron ore , calcite , quartz , illite , chamosita , plaster, brushite and amorphous compounds remain unidentified. Most notable is the presence of a white underground river, liquid milk moon, which is the first global report of this nature.
In the following link you can read a brief chronicle of Ander Izagirre and watch a video with the images of the unique gallery:
Video of the newspaper El mundo