den, cave, excavation
洞 centers on a hollow space underground or in a surface: a cave or den. The excavation sense is the act of making such a hollow.
洞 combines 氵 (water) with 同, which likely contributes the sound and the idea of a hollow or passage. The character suggests a water-worn hollow space.
Water 氵 flowing through a hollow passage 同 creates a cave or den. Picture a stream carving out a deep cave over time.
For ドウ, imagine a deep cave echoing with the sound 'doh' when you shout into it: doh -> ドウ, and the hollow cave amplifies the sound.
cave; cavern; grotto
hollow; cavity; hole; cave
cavity; hollow; cave; cavern
discernment; insight
cave; cavern; den; grotto
limestone cave; limestone cavern
Soto school (of Zen Buddhism)
paper-covered lamp or lantern
wind tunnel
residence of a retired emperor; dwelling of a former emperor
palace of a retired emperor
coronary sinus
coronary sinus
brown-throated martin (Riparia paludicola); brown-throated sand martin
sand martin (Riparia riparia)
druse; globular calcium oxalate crystals, often lining a rock cavity
geode
Soto Zen
tree hollow; hollow in a tree
sinus; cavity; antrum