Dude, great idea for mass destruction ... make a 50 wide sphere (world edit would help)
cover it with glass and a layer of black wool, then make a straight shoot up through the core to the top, put a white wool and put a wooden pressure plate on it ... wala, u have a giant bomb LOOKING TNT Explosive ... and you could just say screw the glass part but i just like to hear glass shatter so :P lol