Most of the cave's walls and ceilings are surprisingly smooth,
which, when combined with the tube's shape, makes for a
surprisingly strong resemblance to "normal" subways.
(Hence the name.)
However, towards the far end of the cave, the ceiling and walls
are sprinkled with small "lava-cicles" -- small spikes of rock
where the dripping lava cooled in place.
The only way to make this shot come out was to shine both
flashlights right along the ceiling and then take the picture.
The built-in flash alone just didn't cut it. ...