The Big Opal is a straightforward way to get underground without committing to a long tour. The self-guided walk drops roughly 30 metres into old tunnels, with enough rough texture left in place to remind you that this was work before it was tourism.
It is a good companion to Chambers of the Black Hand. Do one for the spectacle, the other for the simple feeling of being under the Ridge, looking at the earth that made the town possible.