Okay, Australian Outback holes weren't what I was thinking of. Was mainly thinking about the ability to get a wireless connection underground being pretty rough, especially when it's a cave filled with water. (Sonar based wireless? Sofi?)
Fibre in a cable would work, as would regular ethernet cable - many tourist caves are wired for light show effects.
There are a good number of caves (eg: one former tourist cave on a property I once owned in the WA S'West Karri forrest) that have solution pipes that go straight down from the surface into the roofs of various chambers - they're good for running cable.
Circling back to mining, underground mines can be vast systems of tunnels with custom trucks and trains running about in addition to borehole machines and other stuff - Mining Comms is a whole field with hybrid cable + transmitter (with repeater) hubs, etc.
Not saying this is cheap or easy, but it's all doable - and for the DIY home handy type it can cost time and effort rather than money if they have access to mining auctions | closing down | old stock etc.
I ran into a fellow a while back who was attempting to repurpose in-mountain bunkers as datacenters, so he presumably had some plan for getting the bits in and out — ethernet over power? (the natural water cooling is a plus, and wet limestone sure cuts down on potential covert channels, so maybe there'd be a market among the paranoid?)
look, as a current cave diver and a former devops/survivor of the past 25 years of silicon valley, one of the reasons why diving in the jungle is so delightful is that pagers don't work in the underworld. Now, don't go ruining our last safe spaces.
Somewhere in the long dark ago I saw a human interest story about a hermit who made himself a house in a cave. It was still a cave, but it was clear that someone at least slightly civilized lived there.
Coober Pedy's a long standing still active mining "cave" town - and there are others about the globe, some going back a thousand years and more.