洞 - den, cave, excavation

cave; cavern; den; grotto

cave; cavern; grotto

cavity; hollow; cave; cavern

limestone cave; limestone cavern

discernment; insight

hollow; cavity; hole; cave

residence of a retired emperor; dwelling of a former emperor

Soto school (of Zen Buddhism)

venous sinus

Soto Zen

service tunnel (cables, gas, etc.)

wind tunnel