I’ve just been swallowed by a rabbit hole on heavy hydrogen, the Norwegian hydro plant used to make fertiliser by splitting water and D being left behind, then them realising they could sell D₂O.

It being used in some medicines, and in semiconductor processes, then in preserving Blue OLED for longer.

from the article:

Stable, NON radioactive isotope of hydrogen with one proton and one neutron in its nucleus. Natural abundance of 0.015% of hydrogen atoms on Earth