This is a bit of a trick question! There are many fictional seas, like the Sea of Tranquility on the moon, or the Sea of Monsters from Greek mythology. However, if we're talking about real seas on Earth, it's impossible to say definitively which one doesn't exist.

Here's why:

* The definition of a sea is flexible. Some definitions require a sea to be a large body of saltwater connected to an ocean, while others include large saltwater lakes.

* There are many disputed names. Seas often have multiple names, and some names are no longer commonly used. For example, "The Sea of the Hejaz" is a historical name for the Red Sea.

So, while there's no single sea that definitively doesn't exist, there are definitely many names for seas that are not in common use.