Grammar mavens - a question

Posted by ESC on May 13, 2003

"Coal-mine subsidence means the collapse of an underground coal mine resulting in damage to a surface structure."

I made "coal mine" (the noun) two words because that's the way the United Mine Workers do it. I hyphenated "coal-mine" (the adjective) using the compound modifier rule in the AP Stylebook. Am I right or wrong?