.div1 {
position:absolute;
width:100px;
height:100px;
background:green;
}
.div2 {
z-index:-1;
position:absolute;
left:200px;
top:200px;
width:100px;
height:100px;
background:yellow;
}
.boom {
width:75px;
height:75px;
border-radius:50%;
background:red;
}