At least one-half of all the iron on earth, and in the Universe as a whole, has been synthesized in thermonuclear reactions in the cataclysmic stellar explosions called Type Ia supernovae (SNe Ia). These are the same type of explosions that have served as distance-measurement rulers which, 27 years ago, provided the first indications that the cosmic energy budget is dominated by a still-enigmatic "dark energy". However, we still do not know for sure what is the object that is exploding in a SN Ia, and the details of why and how that explosion occurs ("the SN Ia progenitor problem").
I will review my work in measuring the rates at which SNe Ia explode at different cosmic environments and epochs, and the implications for the progenitor problem and for the universal history of element synthesis.