vault (of sky), dome
窿 centers on a high, curved enclosure: a vault or dome, especially the sky's overarching shape.
窿 combines 穴 (cave, hole) with 隆 (prosperous, high), suggesting a high, hollow space like a vault or dome. The exact historical development is uncertain, but the component relationship is clear.
A cave 穴 that rises high and prosperous 隆 forms a grand vaulted ceiling, like the dome of the sky.
For リュウ, imagine a dragon (竜, りゅう) coiled inside the vault of the sky: 竜 -> リュウ, and the dragon's lair is a high dome.
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vault of heaven; vaulted sky