No, not underground photography but using extension tubes to take macro shots.
A short while ago I spotted a set of extension tubes for sale on eBay. Lightweight alloy by the feel of them. Full electrical contacts are fitted so auto focus works as normal. This set is designed for Canon DSLR cameras and consists of three tubes of different lengths so you can choose which to use depending how close you want to get.
These hand held photos (uncropped) were taken with the 'full spectrum' 1200D set for B/W, 21mm tube with 24-105mm zoom lens. Live view was used on the camera:
A small cactus: (1/100 f8 67mm) middle branch is bout 6mm across
A miniature daffodil: (1/160 f8 92mm)
I was pleased the outdoor daffodil worked as there was a breeze wafting the flowers about.
The advantage of using tubes is no extra optics so no added distortion. The disadvantage is the longer the tube used the more light is lost = slower shutter speeds and / or wider apertures for the same exposure. The 21mm is about 2 f stops slower compared with no tube.
This set seems to be a snip at just under £15 with free p&p.