"Pop up" is a commonly used term that refers to something that suddenly appears unexpectedly. However, there are many other synonyms that can be used to describe this action. Some of these include "arise," "emerge," "surface," "spring up," "crop up," "materialize," "show up," "appear," and "manifest." Each of these terms evokes a slightly different meaning and can be used in different contexts depending on the situation. By using synonyms, we can add variety and depth to our language and communication, and better convey our intended meaning in a more nuanced and effective way.