窟 - cavern

cave; cavern; grotto

cave bear (extinct, Ursus spelaeus)

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

cave diving

The Count of Monte Cristo (novel by Alexandre Dumas)

opium den

cave; cavern

slum

cave temple

den of vice; rogue's den

very shrewd in preparing a means of escape; being very good at escaping danger

cavern; grotto; rock cave