When we say that someone "goes underground," it typically means that they have gone into hiding, either for personal safety or to carry out secretive work. Some synonyms for this phrase might include "disappears," "evades capture," "goes incognito," "lurks," or "vanishes from sight." Other similar expressions could include "withdraws from view," "goes off the grid," "drops off the radar," or "enters stealth mode." Essentially, any phrase that conveys a sense of going into hiding or becoming less visible could serve as a synonym for "goes underground".