洞 - den, cave, excavation

discernment; insight

hollow; cavity; hole; cave

limestone cave; limestone cavern

cavity; hollow; cave; cavern

cave; cavern; grotto

cave; cavern; den; grotto

venous sinus

service tunnel (cables, gas, etc.)

sand martin (Riparia riparia)

sinus; cavity; antrum

vaginal cavity; vagina

palace of a retired emperor