It's amazing how I'd never heard the word "spelunk" before today, and now in the span of the last few hours, I've heard it multiple times in three different contexts.
Curiously, in Portuguese we have "espelunca" which is more commonly used as a synonym for a seedy, shady place -- and now I know why!
Seems there may be a cross-culture notion of caves and sketchy bars having similar level of… ah, “shadiness”. Makes some sense, as I’m typing this out. The dark is where (both literally and figuratively) shady things go down, and it’s hard to get much darker than a cave, so “Spelunke” seems a fitting name.