洞 - den, cave, excavation

limestone cave; limestone cavern

discernment; insight

cavity; hollow; cave; cavern

cave; cavern; den; grotto

hollow; cavity; hole; cave

cave; cavern; grotto

wind tunnel

olm (Proteus anguinus)

making hollow; making meaningless

dongxiao (Chinese bamboo flute similar to a shakuhachi)

sea cave; marine cave

insight; discernment