How do I get rid of these lines in Word?

When I do a row of underscores in Word it turns it into a long thick horizontal line which is impossible to get rid of. I can't delete it, it moves around without me doing anything, and now more lines are appearing! I tried cutting and pasting the text into a new doent, but the lines came too even when I hadn't cut them! Help! How do I delete these annoying lines!