Thats where the processor sits. If u open the back cover u can feel it at the back more directly behind the screen. Have u ever used a smart phone before? I ask cuz I wanted to know like compared to other devices u use how bad is it. Afaik the xperia z also has same heating up problems. Perhaps too powerful of a cpu lol. A good way to combat it is undervolting. Not sure if available for s4 d or not