#Underline text in different lightweight markup languages:

Emacs #OrgMode :orgmode:, Muse:
_this_

#DokuWiki:
__this__

#Textile, #Jira
+this+

There is no Markdown syntax for underlined text, but #pandoc's #Markdown reader treats the content of spans with class "underline" or "ul" as #underlined:

[important]{.underline}
[nota bene]{.ul}