Selective breeding & Pod selection. First find a pepper that has the hottest heat you've found, then save the seeds-grow out next year-find another plant that has a pod that has nuclear heat-save those seeds-grow next year- two selected nuclear plants- cross pollinate- pods produced should be really hot but not always- select a pod that has the hottest heat from all the pods it's produced and save seeds-- continue to grow it until it's stable. It doesn't happen over night-- I think it's about 7+ years to get a stable strain. Most people are focusing on all different characteristics when breeding, taste, shape, texture & heat. Not just heat. (little more to it but you get the point, don't want to clog up this section with grow info)