That is a bit of an odd ball for sure.
I know an SLR is not the most convenient piece of kit to take underground. But these days the entry level Nikon D3400 with 24 meg pixels gives an ISO up to 25000. That tiddler is only one stop faster in reality.
Stick a 1.8 lens on an SLR and it all balances out.
Have to say there is something to be said for using an SLR underground wrapped in a carrier bag and elastic bands trying to keep the moisture and dust at bay ....