There was a recent David Attenborough narrated documentary about a huge cave system in I want to say Vietnam. In the behind the scenes making of short they said drones would not work underground due to some built in programmed restrictions. They had to contact the manufacturer to get the firmware changed to allow it.

Not mentioned here but I believe this is the correct series: https://www.bbc.com/mediacentre/mediapacks/planet-earth-thre...