Cueva de los Verdes (Spanish for "Greens' cave", because in the past its owners were the "Verdes" family) is a lava tube and tourist attraction on the island of Lanzarote. The cave lies within the Monumento Natural del Malpaís de La Corona. The cave was created around 3,000 years ago by lava flows erupted from the nearby volcano Monte Corona, flowing across the Malpaís de la Corona toward the sea. The lava streams cooled on top, developing a solid crust, before the lava drained away leaving the top part as the roof of a cave. In about 20 spots, the roof of the cave collapsed, forming a cavern known locally as a jameo. The caves extend for 6 kilometres above sea level and for another 1.5 kilometres below the sea.
Source and more informations: Cueva de los Verdes - Wikipedia