I caved quite a bit during my teen years, was even a card-carrying member of the National Speleological Society for a few years. While I personally never spent more than three consecutive days underground, that’s more than enough for vertigo to set in. Gravity gives you a general sense of where, “down” is, but the absence of a natural horizon can be disorienting.
I grew up in the California desert and we used to explore abandoned mines when I was in high school. We found an exceptional one that had miles of tunnels and shafts. While not “professionals” or even novices, we spent months exploring it for sheer adventure. I do remember a harrowing trip where we had just descended a shaft and dropped our only light, breaking it. That darkness quickly became material in a way I’ve never experienced since. Luckily we were on a rope in a relatively-known passage, and others were near enough by to find us pretty quickly.
We were very stupid and took idiotic risks, but it was fun. On our last trip, one of our main entrance shafts had completely collapsed during one of the regularly-occurring earthquakes. Despite having wormed our way through worse passages, we finally figured this was probably not the safest pastime. There’s just something visceral about seeing and smelling recently-shattered boulders infill a well known passage that makes even the most math-challenged understand the forces at play. Of course, at least rain and tides were not an issue. LOL