Undersea means happening or existing below the surface of the sea. It is an adjective that lives in front of a noun and almost never anywhere else: undersea cables, an undersea volcano, undersea life, an undersea earthquake.

Undersea這個字 means happening or existing below the surface of the sea意思是發生或存在於海面下。It is an adjective that lives in front of a noun and almost never anywhere else它是個幾乎只會放在名詞前面的形容詞，像undersea cables海底電纜、an undersea volcano海底火山、undersea life海底生物、an undersea earthquake海底地震。

You would not say "the volcano is undersea" — for that, English says "the volcano is underwater" or "under the sea." Compare it with its neighbours. 'Undersea' feels technical and slightly grand; it belongs to news reports, documentaries and engineering — the internet cables that cross the Pacific are undersea cables.

You would not say 'the volcano is undersea'你不會說'the volcano is undersea'——for that, English says 'the volcano is underwater' or 'under the sea.'這種情況英文會說'the volcano is underwater'或'under the sea'。Compare it with its neighbours拿它跟意思相近的字比較一下。'Undersea' feels technical and slightly grand'undersea'給人一種專業、有點正式盛大的感覺——it belongs to news reports, documentaries and engineering常出現在新聞報導、紀錄片和工程領域，the internet cables that cross the Pacific are undersea cables橫跨太平洋的網路電纜就叫undersea cables。

'Underwater' is the everyday word and is far more flexible (underwater camera, swim underwater, it's underwater). 'Submarine' as an adjective means exactly the same thing but is the science word: submarine volcano, submarine cable.

'Underwater' is the everyday word'underwater'是日常用字，and is far more flexible用法也彈性得多（underwater camera防水相機、swim underwater在水下游泳、it's underwater它在水面下）。'Submarine' as an adjective means exactly the same thing'submarine'當形容詞用時意思完全一樣，but is the science word只是它是比較學術的用字，像submarine volcano海底火山、submarine cable海底電纜。

So the three overlap heavily, and the one you pick is mostly a matter of how formal you want to sound. One useful note for Taiwan: the cables that carry Taiwan's internet to the world are called undersea cables or submarine cables in English news — 海底電纜 — so this is a word you will meet in real headlines.

So the three overlap heavily所以這三個字重疊很多，and the one you pick is mostly a matter of how formal you want to sound選哪一個主要看你想聽起來多正式。One useful note for Taiwan對台灣人有用的一點：the cables that carry Taiwan's internet to the world are called undersea cables or submarine cables in English news把台灣網路連到世界的那些電纜，英文新聞裡叫做undersea cables或submarine cables，也就是海底電纜，so this is a word you will meet in real headlines所以這是個你會在真實新聞標題裡遇到的字。