纵深
zòng shēn · HSK 7
depth
纵 (lengthwise) + 深 (deep) = how far something extends front-to-rear
纵 means vertical or lengthwise and 深 means deep, so 纵深 is depth measured from front to back — used for terrain, territory pushed into, or the depth of a military deployment.
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.