真的下面那一面
這一組就是畫面本身，先是動物，再擴大到夠大的東西。共同點是：要從下方看，才會遇到這一面。
noun · B2
The lower part of an animal's body, including its stomach — the side that faces the ground.
動物的腹部下側
核心畫面
An animal seen from below. Its back is covered — scales, fur, shell — but its belly faces the ground, pale and bare. Two things are true of that surface at once: you do not normally see it, and nothing is protecting it. Every use of underbelly leans on one of those two facts, and the strongest uses lean on both.
可以想成從下面看一隻動物。牠的背上有鱗、有毛、有殼。肚子卻朝著地面，顏色偏淡，什麼都沒有。這一面同時有兩個特點。第一…更多收起
可以想成從下面看一隻動物。牠的背上有鱗、有毛、有殼。肚子卻朝著地面，顏色偏淡，什麼都沒有。這一面同時有兩個特點。第一，平常看不到。第二，完全沒有保護。underbelly 的每一種用法，都靠著其中一個特點；最有力的那幾種，兩個一起用上。From the word’s history
Wiktionary gives the formation as under- + belly. The word is built, not borrowed, so the image is already in the parts.
Wiktionary 記載這個字就是 under-（下面）加 belly（肚子）組成的。它不是外來字，而是英文自己拼出來的，所以畫面本來就寫在字裡：動物翻過來才看得到的那一面。
Wiktionary entry for underbelly (kaikki extract, source_ref kaikki:3f6f69a190036ae4:1176516766), etymology_text: From under- + belly
Three branches
平常看不到的那一面
只取畫面裡的第一個特點：這一面朝下、藏著，不翻過來就看不到。翻的人通常是記者、攝影師或紀錄片。
noun · C1
The ugly, unpleasant side of a place, a group or a way of life — the poverty, the crime, the unfairness that is usually kept out of sight.
陰暗面
最沒有防備的那一塊
取畫面裡的第二個特點：肚子上沒有殼。既然沒有保護，攻擊那裡最有效，所以這個字自然變成「要害」。
noun · C1
The weakest part of a person, a group or a plan — the part with the least protection, which is easiest to attack or criticise.
最沒有防備的弱點