At the foot of the Corona volcano, the island's subsoil is riddled with underground galleries. Upon contact with the air, the surface of the lava streams cools and hardens into a dense layer of basalt, underneath which molten magma continues to circulate. The Cueva de los Verdes galleries, said to be 5 000 years old, run for 7km to the coast. A beautifully illuminated 2km-network of these tunnels runs along various levels.