This article is about the reflective water droplets often seen just inside cave entrances.
After you enter a cave and become dark adjusted, you can sometimes
see what appears to be droplets of condensation on the walls.
These are only visible with a cap mounted lamp (eg miners' light or Petzl Zoom). If you look at them closely you can see that they are spheres of water on an otherwise dry surface. They range in size from about 0,1 mm to about 2 mm diameter. The size of the droplets varies according to the amount of available moisture, eg during dry spells the droplets may disappear entirely whereas after a thunderstorm the droplets may be well developed. They are usually silvery but sometimes take on gold, yellow and brown tones. Sites with reflective droplets usually persist from year to year unless disturbed.
Where are they found in caves?
They are usually found on walls and ceilings, usually within about
20 metres of the cave entrance.
They are frequently associated with areas of white pasty deposits
like moonmilk. They also occur on limestone bedrock which has a fine grey
They occur in the twilight to dark zones of caves.
They occur in lava tubes.
A good display can be seen in the roof of the Exhibition Chamber of Lucas Cave at Jenolan Caves.
Why were they not reported a long time ago?
A long time ago people did not use cap lamps; light sources were hand held. The droplets are not visible until the light source is close to the eye (eg about 10 to 20 cm away). Also in the not-so-distant past people just assumed that the droplets were condensation and that was that.
What are they? Isn't it just condensation?
The grey coating on the limestone bedrock, where the droplets occur, is a colony of microorganisms called actinomycetes. These are common soil microorganisms which occur all over the world. It's not just one species, there are a few living together as a symbiotic colony. Mostly the surface of the colony is hydrophobic (repels water) but it appears to develop filamentous structures which are hydrophilic (attracts water). It is this filament to which the spherical water droplets are attached. You usually see them in a group, rather than one or two.
How can I tell whether the dots are condensation or not?
True condensation droplets are usually quite large, rather like the drips on
straws. Also when condensation occurs, usually the cave walls are dripping
Where the reflective drops occur, the walls are usually relatively dry.
Condensation droplets are about 5 mm diameter whereas the reflective dots are typically from 0,5 to 2 mm diameter.
Coloured Reflective Dots - what are they?
There are two sources of colour: one is simply a lens effect over whatever colour the substrate is. The other is the colour of the droplets themselves. At certain times of the year, usually associated with autumn, the droplets can become a golden colour. The colour was found (ref: Moore and Sullivan) to be due to beta carotene. It is assumed that this has something to do with the microorganisms' reproduction since this time of the year is also the best time for common forest fungi to reproduce (relatively warm soil temperatures with high humidity).
What are they useful for?
One of the best uses of these reflective dots is in cave exploration.
If you see colonies of reflective dots deep in a cave, there is most
likely an entrance nearby. This can also be useful for determining air
flow patterns and other cave climate measurements as well as a purely
useful tool for locating alternate entrances.
Another good use for them is in the exploration of lava tubes. In some tubes the basalt walls are so dark that exploration is difficult (absorbs ones light). With a large colony of these reflective dots however, one can easily see the extent of the roof and walls with only a small light source. An example of this is in Harman One Cave near Byaduk (Victoria) and the small Tunnel Cave at Mount Eccles (Victoria).
If you look closely at these colonies with a hand lens you often see different coloured patches. Presumably these are the different microorganism species forming the colony. You can also see the filaments to which the reflective droplets are attached. You may also see small colonies developing off the main colonies. These are most prevalent just after periods of colouration (hence the theory about colouration and reproduction).
Tourist Do's and Don'ts
- Do put the flash close to the camera to take photos of the colonies.
- Do keep a record of where you see them, when you see them, what time of the year and what colour they were.
- Don't go round poking them. Treat them the same way as you would some piece of artwork: admire and look but don't touch.
- Take care when caving so as not to disturb them as you move through a cave. Try not to brush against them as it spoils them.
- Note to tourist cave owners / operators: Be careful during cave cleaning operations. It is probably not a good idea to steam clean areas of reflective dots as it may kill the microorganisms. I would expect the colonies to cope with mild amounts of dust and lint by growing over them. Their location near the cave entrance probably infers that they obtain some of their nutrients from airborne particles.
Where else do they occur?
They also occur under rotting timber in forests as they are a common soil organism. Neville reported seeing such colonies under rotting wood; We have also seen them in Switzerland in Hölloch Cave (near the entrance) and in the cave of Jean-Jacques Rousseu (in the main chamber).
How do they get there?
Most likely they are deposited from airborne spores, which would adhere to the cave walls when air movement is from outside to inside the cave.
Most likely in a pristine cave, these colonies would also occur on the
floor of the cave. However that is where they would get trampled by
people and wildlife, or flooded out at times so they rarely occur on floors.
I have seen them on the floor in sheltered areas of dry caves occasionally.
Possibly these microorganisms contribute to the "clean earth smell" noted by cavers as they enter / exit caves.
Some of the water collected from these droplets was analysed to see what minerals were present. The only mineral detected was calcite. Possibly the presence of these microorganisms contributes to deposits of moonmilk near cave entrances.
I don't know whether the droplet is condensed onto the hydrophilic surface from the air or whether the colony absorbs water and deposits it on the filaments.
Personal comments with Neville Michie from 1997 to 2001
Speleology by George Moore and Nicholas Sullivan.
Cosmetic update, January 2006. Content updated 27th December 2001.