Talk to Bill Stone about that - he's been one of the preeminent cave explorers and they've been working towards exactly that: https://stoneaerospace.com/ - look at Prometheus and Archimedes in particular.

And Enceladus, too! I've been reading about it lately, and I would be quite surprised (not to say I'm anywhere close to an expert) if there wasn't life there. It's got a rocky core surrounded by a deep ocean surrounded by a layer of ice. It's core gets a geological workout from tidal forces from Saturn (i.e., heat), and there's clear evidence of volcanic activity from the plumes that are ejected.

I'm a little surprised we don't more readily explore Europa and Enceladus. They are far more interesting than Mars.

I am not an expert in the field and happy to be corrected, but i think Europa is vastly underrated and i agree with your comment. I suppose the challenge is the thickness of its crust but in my view its worth exploring the options to reach its water ocean and potentially explore a research base. From water one can generate fuel, oxygen and do farming. Really wish someone knowledgeable would chip into why this moon is not getting more attention.

What about the tentacles?

http://allreaders.com/book-review-summary/2061-odyssey-three...

One of the side effects of Spacex Starship, if it works, is it'll make it was easier and cheaper to send a bunch of stuff out into the solar system.

How difficult would it be to drill down through the ice? Would melting through the ice be a viable alternative or would that end up being less efficient?

If we focus on life extension technology we could do a lot more in your lifetime.