*{ margin:0; padding:0;}
.bg{ width:100%;}
.btn{ width:150px; height:50px; background:#CCC; text-align:center; line-height:50px; margin:0 auto}
.box{ width:500px; height:300px; background:#CCC; margin-left:-175px; display:none}
.btn:hover .box{ display:block}