The XENON1T detector has allowed scientists to observe an ultra-rare event: the radioactive decay of xenon-124.

 

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In a subterranean laboratory, about a mile below Italy's Gran Sasso mountains, scientists are hunting for dark matter using an incredibly powerful detector full of liquid xenon. In their search for the elusive particle, they observed something else entirely.

Their dark matter detector witnessed the rarest event ever recorded: the radioactive decay of xenon-124.

https://www.cnet.com/google-amp/news/dark-matter-scientists-observe-the-rarest-event-ever-recorded/?utm_source=quora&utm_medium=referral

Serendipity.......

About a hundred times as common as this.
http://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=2240&context=chem_facwork