body {
  background-image: url(index.png);
  background-repeat: repeat;
  padding-left: 50%;
  padding-top: 15%;
}

#underground {
   position:absolute; 
   left:50%; 
   top:50%; 
   width:30em; 
   height:20em; 
   margin-left:-15em;    /* width/2 */ 
   margin-top:-10em;    /* height/2 */ 
   padding:10px; 
   overflow:auto;
}