Cassandra Data Center can be multiple in number and can be Geographically Distributed :)

https://www.instaclustr.com/around-the-world-globally-distributed-storage-streaming-and-search-an-introduction-to-cassandra-multi-data-centers-part-2/#

https://www.instaclustr.com/around-the-world-in-approximately-8-data-centres-globally-distributed-storage-streaming-and-search-part-1/#

https://www.datastax.com/blog/deploying-cassandra-across-multiple-data-centers

https://www.slideshare.net/DataStax/apache-cassandra-multidatacenter-essentials-julien-anguenot-iland-internet-solutions-c-summit-2016