Photos

This image was taken using an open shutter - the flash was triggered manually while walking in the tube.
The lines of light are made by a torch.

I also post as a workshop an image with the flash mounted on the camera - the 3D look is gone...



from wikipedia:
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like channel.

Lava tubes can be up to 14-15 m wide, though are often narrower, and run anywhere from 1-15 m below the surface. Lava tubes can also be extremely long; one tube from the Mauna Loa (Hawaii, USA) 1859 flow enters the ocean about 50 km (over 30 miles) from its eruption point, and the Cueva del Viento - Sobrado system on Teide (Tenerife island, Spain) is over 18 km long, due to extensive braided maze areas at the upper zones of the system.

Lava usually leaves the point of eruption in channels. These channels tend to stay very hot as their
surroundings cool. This means they slowly develop walls around them as the surrounding lava cools and/or as the channel melts its way deeper. These channels can get deep enough to crust over, forming an insulating tube that keeps the lava molten and serves as a conduit for the flowing lava. These types of lava tubes tend to be closer to the lava eruption point.