窟 - cavern

cave; cavern; grotto

brothel; house of ill repute; red-light district

enchanted cave

treasure mine

The Count of Monte Cristo (novel by Alexandre Dumas)

opium den

cavern; grotto; rock cave

cave; cavern

den of vice; rogue's den

cave bear (extinct, Ursus spelaeus)

buried earthen jar that makes sound when water drips into it (garden feature)

cave painting