At InterDrone ’17, I observed a nicely protected drone designed for hazardous conditions from FlyAbility (http://www.flyability.com), asked about use in caves. Logical, right? As I recall the conversation, apparently the person I spoke with did not think it easily feasible to use in caves, because the transmission (for controls and for video) would choppy. The again, the drone could become lost on the return trip.
When I suggested the drone drop repeater antenna along the way, or unspool a light transmission wire, you see a lightbulb coming on in their heads.
With a suitable drone platform, you could both unspool the wire, and re-spool it as it shows the path out. Or you could drop antenna with ‘hot, infrared markings and scoop them back up with a “V” trap on the way back. Or re-spool antennae that were dropped with a thin line attaching them all….