
unterirdisch
underground
Located below the surface of the earth or taking place there.
Die U-Bahn fährt durch ein unterirdisches Tunnelsystem.
The subway runs through an underground tunnel system.
underground / abysmal
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underground
Located below the surface of the earth or taking place there.
Die U-Bahn fährt durch ein unterirdisches Tunnelsystem.
The subway runs through an underground tunnel system.

abysmal
Colloquial for extremely bad or of very poor quality.
Die Leistung der Mannschaft war heute einfach nur unterirdisch.
The team's performance today was simply abysmal.
The word is made up of 'unter' (under) and 'irdisch' (earthly, from 'Erde' meaning earth). So it literally means 'located under the earth.'
Archäologen haben eine unterirdische Kammer entdeckt.
Archaeologists discovered an underground chamber.
You can easily recognize the meaning from the context. If people are talking about places, buildings, or passageways, the literal meaning is intended. If it is a strong negative evaluation of quality or performance, the colloquial meaning is intended.
Viele Städte haben große unterirdische Wasserreservoirs.
Many cities have large underground water reservoirs.
In the meaning 'extremely bad,' 'unterirdisch' (abysmal) is a very strong colloquial word. It expresses great disappointment or annoyance and is mostly used in informal conversations.
Seine Entschuldigung war absolut unterirdisch und hat alles nur schlimmer gemacht.
His apology was absolutely abysmal and only made everything worse.
As a negative evaluation, 'unterirdisch' (abysmal) is often used for abstract things. Typical phrases are 'unterirdische Qualität' (abysmal quality), 'eine unterirdische Leistung' (an abysmal performance), or 'ein unterirdischer Film' (an abysmal film).
Die Bildqualität der alten Kamera ist aus heutiger Sicht unterirdisch.
The image quality of the old camera is abysmal by today's standards.
bad
Schlecht and unterirdisch both refer here to low quality. Schlecht is the neutral, broader assessment. Unterirdisch is colloquial and strongly emphatic; it fits when something is considered extremely bad.

Der Service war unterirdisch, denn den ganzen Abend bediente uns niemand.
The service was abysmal because no one waited on us all evening.
Der Service war schlecht, denn wir mussten sehr lange warten.
The service was bad because we had to wait a very long time.