Dixit Wikipedia: Lava tubes are formed when an active low-viscosity lava flow develops a continuous and hard crust,which thickens and forms a roof above the still-flowing lava stream. Tubes form in one of two ways: by the crusting over of lava channels, and from pahoehoe flows where the lava is moving under the surface.

This lava tube is in two parts: a first one which was cleaned and lighted and the second one which is more ``natural''.