深まる
ふかまる · fukamaru · JLPT N3 · verb-godan
to deepen
🌊 深い fukai (deep) + -まる (becomes so by itself) = to grow deeper
The -まる ending makes an intransitive verb: the thing deepens on its own, as in 秋が深まる (autumn deepens). Its partner 深める fukameru is transitive: you deepen something. The same pair shows up in 高まる / 高める (to rise / to raise).
深 — water radical 氵 + 罙 (phonetic, depth)
深 combines the water radical 氵 with 罙, which depicts a hand reaching into a cave or hollow. Originally it described deep water, then broadened to mean 'deep' in any sense — distance, color, or feeling. The 氵 gives the meaning, while 罙 hints at both sound and the idea of reaching far in.