The sensory deprivation alone is enough to drive me nuts thinking about it.
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 am claustrophobic, so I don’t even want to think about it.