How Do I Do “Underline” On A WP Page?

I have got a Word Press website and are adding a few new pages to it about SEO, I just noticed though that there is no button to make text Underlined, there is Bold, Italics, etc.. buttons, but no Underline one. I also tried changing the editor so i can see the code and added the <u> </u> tags, but it doesn't work and just removes the tags. Can someone let me know why this is? and also How do I get the Underline working? Thanks!

See original post by Sean101