"Go underground" is an idiom that means to hide or go into hiding. There are various synonyms for this phrase, such as "disappear," "conceal oneself," "go into seclusion," "take cover," "become invisible," "go into hiding," "lay low," "stay out of sight," "go incognito," and "go undercover," among others. These phrases commonly refer to concealing oneself from danger, law enforcement, or unwanted attention. Each synonym holds a slightly different connotation and can be used in different contexts depending on the situation. It is essential to choose the correct phrase to convey the intended meaning accurately.