Why after the prepostion "unter" does the dative follow it?

[May_34]

I am very confused why after "unter" you use the dative case .
For example:
Unter dem Meer
Why after that does the dative follow.
Please can anyone explain this to me. Thank you

[sweilan1]

Unter takes either accusative or dative depending on usage.

https://www.thoughtco.com/two-way-doubtful-prepositions-in-german-1444444

[slamRN]

Das ist ein guter Link. Danke!