St. Paul in the late 90s, there was a series of underground (literally) parties, known as the "rave in a cave". They took place in the Wabasha caves: https://en.m.wikipedia.org/wiki/Wabasha_Street_Caves

The entrance to where the parties were being held has been sealed, but there are other various entrances near the river. Every few years some kids get stuck, and the city tries to restrict entrance. Caves are just too interesting.