From Bob Masta, Ann Arbor, Michigan, US
So drones could be used to map unsafe mines (22 April, p 16). But how can they can be remotely controlled once they turn a corner, given that radio waves don't penetrate rock or earth very well?
Cavers have been working on this problem for decades. The best scheme that I have heard so far involves planting walkie-talkie-sized repeaters every 100 metres or so along passages. This achieves data transfer at text-messaging speed – too slow for navigation and control.
