Probably the most familiar is kryptonite (it was later redefined as an alloy, but started off as a classic 'element unknown to Earth science'). This is why I just love the sound of denatonium benzoate. Surely denatonium must be stronger than any known material, or has tendency to leave superheroes weak at the knees? But no, this is a compound that makes things taste extremely unpleasant - and it's the subject of my latest podcast for the Royal Society of Chemistry.
Wash your mouth out and hurry over to the RSC compounds site to see more on this incredibly bitter product that has the capacity to save lives. If you'd like to listen straight away, just click here.