Cueva De Los Verdes

The Green Caves.
More photos here.

"La Cueva de los Verdes is located in the northern municipality of Haria and integrated into the vast volcanic landscape named Natural Monument of "Malpaís de la Corona".
This formation is the result of the eruptive activity of La Corona Volcano that caused an extensive underground volcanic tunnel of over six kilometres long that runs from the cone of the volcano down to the sea. A volcanic tunnel is formed by the cooling and solidification of the superficial lava flow in contact with air whilst the liquid magma underneath continues flowing. The partial collapse of a section´s tunnel roof forms the structure known in Lanzarote as "Jameo" - an opening that provides access to the other grottoes."