I come from Yorkshire and I’m used to limestone caves, so I was surprised when I found that the volcanic island of La Palma has lots of caves too.
Volcanic caves are formed when a river of lava solidifies on the top and sides, but the middle (insulated by the solid-but-still-hot lava around it) stays runny. Sometimes big bubbles of gas force their way to the surface, leaving a hole which reminds me of boiling porridge which got flash frozen.
The lava in the centre keeps on flowing, and often flows right out of the bottom, leaving a hollow tube of lava. Some of these caves are several kilometres long. Then the lava contracts as it cools, and quite often, sections of roof fall in. These are often the way to get in.
Once the main cave has formed, water seeps in and sometimes leaves lovely little snowflakes of salt on the ceiling.
Please only explore these caves with a qualified guide.
For one thing, many of the caves have very fragile ecosystems. You see, there isn’t much to eat in the parts of a cave which get no light, so the invertebrates evolve without spending calories to create useless body parts like eyes, and pigmentation. Since the temperature and humidity are extremely stable down there, the mini-beasts have thin exoskeletons which offer little protection against changes in temperature and humidity. If you hold one in your hand for thirty seconds, the poor thing dies of heat-stroke.
For another, Todoque cave itself is a protected National Monument. It’s illegal to take anything except photographs or to leave anything but footprints.
And most importantly, the guide knows how to keep you safe. Sometimes the way out is not obvious.