KanjiJLPT N1
Cave

den, cave, excavation

Mnemonic

A violent storm batters the mountainside, but deep inside the cave, everything remains the same — still, silent, untouched. Howling winds can't reach these dark chambers. No matter how many storms rage, the cave keeps that same ancient calm, century after century.
Additional thoughts
Picture a storm pounding a cliff face while you sit peacefully inside a cavern where nothing ever changes — the contrast makes the cave memorable. Storm (water radical) + Same = Cave.
Quick recall
A storm rages outside, but inside the cave everything stays the same.

Details

The keyword for 洞 is cave. This kanji refers to a hollow space within a rock face, mountain, or underground formation — a natural cavity or cavern. By extension, it can also convey the idea of seeing through something or having deep insight, as if penetrating into a dark, hidden space to perceive what lies within. This sense of thorough perception connects the physical notion of a cave with the intellectual concept of piercing understanding.
On'yomi
どう
Kun'yomi
ほら

Used in vocab