As workers search for trapped miners in Utah, having drilled a 9” hole down to what is hoped to be their area, they plan to use things like sound and detecting CO2 and O2 in the atmosphere to find the miners.
It occurs to me that it should be possible to fit one of those inflatable radio controlled blimps down such a small tube, inflating it after it gets to the bottom. There are models that support small video cameras (and LED lights would not be too hard) especially in the denser air at the bottom of a mine. You would send down a radio relay station as well, and if things were really fancy, a way for the blimp to be told to dock for recharge or exchange of battery packs. (Small butane motors might also provide better power for weight.)
It’s also possible that power could be provided by paying out a wire, if it could generate enough thrust to drag that wire. There is a high risk the wire could get caught except on smooth floors, though. One might imagine paying out wire as far as one can go, and then disconnecting, fully charged, for a modest time on internal power. These blimps are cheap, you could send down several. They could easily sail over debris a ground based robot could not handle, though they could not crawl through small holes without deflating.
Another option would be an enclosed fan hovering robot. Such a robot would be able to go through smaller holes, though it’s hard to imagine remote pilots good enough to send them through such channels with only a video camera to see by. In the future, we may well have hovering robots able to use sonar to keep themselves stable and away from obstacles. They would go on ground when they could, then use bursts of hover to get over obstacles. But the blimp is something that could certainly work in ordinary mine channels today, though only for a limited battery life.