This is the 7767th most frequent Polish word.
mariupol
"Mariupol" is not a Polish word; it is the name of a city in Ukraine.
In this sentence, 'Mariupol' is a proper noun used as the name of a city.
Mariupol to miasto położone nad Morzem Azowskim.
Mariupol is a city located on the Sea of Azov.
Here, 'Mariupola' is the genitive case of the proper noun 'Mariupol', since it's used to describe possession ('history of Mariupol').
Historia Mariupola jest bardzo interesująca.
The history of Mariupol is very interesting.
In this sentence, 'Mariupol' is a proper noun in the accusative case, following the verb 'odwiedziłem' (I visited).