Underground oh-no beacon

If you were suitably insane, you might want to go exploring caves or other questionable underground spaces. As extreme sports go, it seems to be quite an efficient way to die, in part because if you get into difficulty, you're a long way underground, and from outside nobody can see that anything is wrong.

What about a beacon one could leave at or near the cave entrance? It has GPS, a timer, and phone reception. Set the timer for the length of time you plan to be in the cave. If you're not out in time to turn it off, it sends a message to a list of people, telling them where it is and that you've not returned on schedule. Maybe it gives half-hourly updates until you get back and cancel it.

Optional features:

a) "GPS, a timer and phone reception" - also known as a phone. This could be implemented as an app, then just leave a spare phone somewhere it has reception, sealed in a ziploc bag with a power pack.

b) For the truly mad who are going cave diving, implement this as a bespoke product. Plug in an underwater microphone on a lead long enough to reach from cave entrance to water. Divers carry an audio alarm that creates a loud sound at a fixed low frequency. This need not necessarily be battery powered; it could be a single-use unit powered by compressed air or, I dunno, dynamite, as long as it creates a unique signature. Power limited by not concussing the user. Once this signature is detected by the microphone, it calls for help, hopefully before the user drowns.

And yes, the latter in particular is quite a lot of complexity for a solution that might not be loud enough to work and probably wouldn't be required even once in the lifetimes of a hundred cave divers.