Back around 2000, an English ham radio operator John Hey, G3TDZ developed a specalized radio for use in cave rescues. "Normal" radio frequencies do not go through earth very well - the higher the frequenices, the more they are blocked. Normal ham radio bands range from around 3 million Hz up to 440 million and up. John's device operated at 87 thousand Hz and is therefore able to penetrate earth and rock.
The HeyPhone was used in the Thai cave rescue. Here is the website: HeyPhone Cave Rescue Communication System
Here is an article on various cave radio systems from 2001: The HeyPhone Story
Fascinating technology. Works so well it has not been changed in 18 years.
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