For the Thailand rescue of the children’s soccer team, it appears they were aided by a device invented by a UK Ham Radio operator.
The “Heyphone” is a voice radio designed by John Hey, G3TDZ, as open-source hardware specifically for cave rescues. It uses upper (single) sideband voice on 87 kilohertz in the LF (low frequency) band.
The antenna consists of two stakes driven into the ground about 20m apart. Enough of the current between them fringes outward to couple to another antenna up to a few hundred meters away (or down).
John Hey passed away in 2016 so he didn’t get to see his work used here. But he made a difference in this rescue and perhaps in many other cave explorations. Thank you John!