I believe this would be a good suggestion to make caves feel more realistic.
A moisture effect, when underground about 50-100 blocks you would start to see a moisture effect form and little droplets of water will start falling from the rooftops of caves/mines.
This would also make your blockhead sweat a little, but no major damage done.
The droplets will also create a slight sound.
Then you are at diamond level the moisture will get intense making your blockhead sweat very bad, and eventually start losing 2 hitpoints/health every minute