
untergehen
to set
Referring to celestial bodies such as the sun or moon: to disappear at the horizon.
Wir saßen am Strand und sahen zu, wie die Sonne im Meer unterging.
We sat on the beach and watched as the sun set into the sea.
to set / sink / perish
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to set
Referring to celestial bodies such as the sun or moon: to disappear at the horizon.
Wir saßen am Strand und sahen zu, wie die Sonne im Meer unterging.
We sat on the beach and watched as the sun set into the sea.

to sink
To sink in water and not come back to the surface.
Das Schiff ist im Sturm untergegangen.
The ship sank in the storm.

to perish
To cease to exist; to be destroyed or come to ruin. Often refers to empires, cultures, or systems.
Viele glaubten, dass die Welt im Jahr 2000 untergehen würde.
Many believed that the world would perish in the year 2000.

to get lost
To go unnoticed or be missed in a large crowd or mass; to get lost.
Bei der lauten Musik ging seine leise Stimme völlig unter.
With the loud music, his quiet voice got completely lost.
'Untergehen' (to set/sink/perish) is a separable verb. In a main clause, the prefix 'unter' goes at the end. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Die Sonne geht heute besonders schön unter.
The sun is setting especially beautifully today.
The verb is made up of 'unter' (under/down) and 'gehen' (to go). The literal meaning 'nach unten gehen' (to go downward) helps you understand most meanings: the sun goes below the horizon, a ship goes under water.
Wir sahen zu, wie die Sonne langsam am Horizont unterging.
We watched as the sun slowly set on the horizon.
The perfect tense of 'untergehen' (to set/sink/perish) is always formed with the auxiliary verb 'sein' (to be), because it describes a change of state or movement. So it is 'ist untergegangen' (has set/sunk/perished).
Das Römische Reich ist vor langer Zeit untergegangen.
The Roman Empire perished a long time ago.
The meaning of 'untergehen' (to set/sink/perish) can often be recognized from the subject. 'Die Sonne' (the sun) sets (disappears at the horizon), 'ein Schiff' (a ship) sinks, and 'ein Reich' (an empire) perishes (is destroyed).
Viele alte Zivilisationen sind untergegangen.
Many ancient civilizations have perished.
In the figurative meaning 'nicht bemerkt werden' (to go unnoticed), 'untergehen' (to get lost) is often used with the preposition 'in' + dative. One gets lost 'in der Menge' (in the crowd), 'im Lärm' (in the noise), or 'in der Masse' (in the masses).
Seine leise Bemerkung ging im allgemeinen Gelächter unter.
His quiet remark got lost in the general laughter.
to overlook / survey
Übersehen describes a person failing to notice someone in a crowd. Untergehen describes the same situation from the perspective of the unnoticed person: they do not stand out within the large group. The words therefore focus on different participants.

Der stille Schüler ging in der großen Gruppe unter.
The quiet student got lost in the large group.
Die Lehrerin hat den stillen Schüler in der großen Gruppe übersehen.
The teacher overlooked the quiet student in the large group.