The word 隧道 is formed through a logical, pictorial combination of two characters. The first character, 隧 (suì), depicts the concept of a passage through a barrier, its left side (阝) is a "mound" or "hill" radical, and its right component historically conveys a sense of "following through". The second character, 道 (dào), simply means "road" or "path". When combined, they create a direct, almost visual composite meaning: a road that passes through a hill or mountain, perfectly defining a tunnel.