I have received an initial enquiry to shoot some footage descending down a large shaft and into an underground tunnel (in the UK) but as yet don't have any specifics on diameter, construction, location, etc. I've asked for more information but in the meantime would welcome advice from anyone that may have done this. - Should I even consider it ??? - What is the minimum diameter for safe flying (4m?) - Flight mode: I'm thinking Atti as P-Opti probably wouldn't work in that environment anyway - Likely to be lots of steel buried in the walls so another reason to fly Atti - Is failsafe RTH automatically disabled if no home-point can be set? Also, where does this fit with the CAA regulations? I've never really thought about this before but do they apply when flying a drone 'indoors' which in effect is what I'd be doing? I have my CAA PFAW. Appreciate any helpful comments.