Thanks to the XENON1T dark matter detector lodged under the Gran Sasso mountains of Italy, scientists have recorded one of the rarest events to ever be detected: a special type of radioactive decay in xenon-124.

It's an amazing feat, because the decay of this isotope is extremely, extremely slow. In fact, xenon-124 has a half-life of 1.8 x 10 to the power 22 years – roughly one trillion times longer than the age of the Universe.

In radioactive decay, half-life refers to the amount of time it would take for half of the atomic nuclei in a given sample to spontaneously change through one of the many types of radioactive decay, which often involve spitting out or capturing protons, neutrons, and electrons in various combinations.

In this case, a team of researchers managed to observe a special event called a double-electron capture, where two protons within a xenon atom simultaneously absorbed two electrons, resulting in two neutrons – described by the team as "a rare thing multiplied by another rare thing, making it ultra-rare".

This exciting observation took place thanks to XENON1T's incredibly precise calibration - the instrument is designed to detect interactions of hypothetical dark matter particles with atoms in the 1,300 kilograms (2,866 pounds) of xenon isotope packed into the tank of the device.

But in this case, the sensors designed to observe such interactions captured the decay of the isotope itself, leading to a rare observation of a different sort.

"We actually saw this decay happen," says one of the researchers, Ethan Brown from the Rensselaer Polytechnic Institute (RPI) in New York. "It's the longest, slowest process that has ever been directly observed, and our dark matter detector was sensitive enough to measure it."

"It's amazing to have witnessed this process, and it says that our detector can measure the rarest thing ever recorded."

Scientists have never before directly observed the radioactive decay of this xenon isotope, though its half-life has been theorised about since 1955. It represents direct evidence of something we've been looking for for decades.

What's actually happening is XENON1T is detecting the signals given off by electrons in the atom rearranging themselves to fill in for the two that were captured in the nucleus. As Gizmodo reports, it doesn't quite hit the statistical threshold to count as a discovery, but it's still an incredible observation.

"Electrons in double-capture are removed from the innermost shell around the nucleus, and that creates room in that shell," says Brown. "The remaining electrons collapse to the ground state, and we saw this collapse process in our detector."

Even though XENON1T was built to look for dark matter, it shows how these instruments can lead to other important findings too. This latest observation could teach us more about neutrinos, abundant but hard-to-detect particles scientists have been hunting for decades.

In this case the researchers saw a two-neutrino double electron capture - the result of the rearrangement of electrons meant two neutrinos were emitted by the atomic nucleus. The next challenge they want to take on is to detect a neutrinoless double-electron capture – an event that's even rarer than this one.

That, in turn, could help unlock some of the deepest secrets of particle physics.

"This is a fascinating finding that advances the frontiers of knowledge about the most fundamental characteristics of matter," says Curt Breneman from RPI, who wasn't directly involved in the study.

"Dr. Brown's work in calibrating the detector and ensuring that the xenon is scrubbed to the highest possible standard of purity was critical to making this important observation."

The research has been published in Nature.