The antonyms for the word "over passing" are underpassing, passing beneath, passing under or passing below. These words all have in common the idea of movement that goes down instead of up or over, suggesting a sense of closeness to the ground or a lower level. Rather than crossing something, these antonyms suggest the act of passing underneath it. In the context of transportation, these antonyms can refer to tunnels or subway systems, but they can also be used in a figurative sense, such as when talking about a person who is "under the radar" or "flying under the radar" of others' attention.