I'm all for doing the unexpected on my vacation. People expect you to stand upon the earth's crust and take in some rays, or maybe they expect you to climb to a high point and look down upon the earth.
I have some contrarian suggestions.
You know you can peer into European volcanoes. You can hike up Mount Vesuvius and peer into the cone, for example. Or go to Taormina in Sicily and get fantastic views of the coast and Mt. Etna volcano. But that just scratches the surface.
Why not go Under Rome? Of course that's only a few meters underground...
To get deeper, why not go to Iceland and start on your journey towards the center of the earth? Then you can tell all your friends you've been inside the Thrihnukagigur volcano. Like the US Congress it's dormant, so you don't have to worry about anything happening.
There are better things to do than roasting in the sun you know.
Oh, and by the way, if you've ever wondered how it is to be one of those people who digs the earth with tiny little tools, there is a free online course you can take, provocatively titled: Archaeology's Dirty Little Secrets.