See example at http://incubator.wikimedia.org/wiki/Wp/shh/Ba%CC%B2i%CC%B2ngwi .
The Shoshoni language (and some African languages) use the underline as a diacritic (see Underscore entry on the English Wikipedia for more details). The unicode used to perform that is the "combine low line", as in the example: a̲i̲. See the description of the unicode character at http://www.fileformat.info/info/unicode/char/332/index.htm .
The problem is in the title of the page the underline is not directly under the letters it should be, but it shifted a bit to the right.