Unable to answer "What kind of memorial is this?" – Mundloch des Caroliner Erbstollens (Memorial) – https://osm.org/node/3326782603 via StreetComplete 57.1:

Erbstollen

Attached photo(s):
https://westnordost.de/p/204026.jpg
https://westnordost.de/p/204027.jpg