深 shows water 氵 beside a figure probing downward with a torch into a cave — the old form drew exploration into darkness. Depth is where you must feel your way.
深 goes down and in: 深水 (deep water), 深夜 (deep night — the honest hours), 深圳 (Shenzhen — 'deep drainage channel', now the tech capital), 深刻 (profound — 'carved deep'). Feeling runs 深 too: 深爱 (love deeply), 印象很深 (deep impression). Chinese compliments thought by depth, not height: a profound idea is carved 深, not built tall.
Wading 氵 past where light reaches — DEEP.
深圳 'deep channel' was a fishing town of 30,000 in 1980; now 17+ million people. The name aged perfectly.