Lots of drones already out there.
The Flyability Elios has been around for a few years for difficult access. It's a drone in a cage so can bounce off stuff. However at £30K for its latest manifestation it's a bit high for most.
The Swedes and Canadians have developed underground drones which use Lidar to find their way around. Great for survey work.
There is an American captive 'drone' which can be lowered down shafts on a wire and can maintain it's position precisely for shaft inspection, etc.
COMRU are looking at a drone which can carry both a gas detector and a thermal camera and can 'remember' where obstacles are as well as turn itself the right way up if it crashes.
Lots going on in the world of underground drones.
The finest workers in stone are not copper or steel tools, but the gentle touches of air and water working at their leisure with a liberal allowance of time.
Henry David Thoreau