> It's thought that you can avoid this by subbing out the helium for hydrogen, but nobody so far has been insane enough to try.

It's reportedly been done very recently using a rebreather (https://gue.com/blog/n1-the-inside-story-of-the-first-ever-h...).

Prior to that, in 1992, there was the French COMEX-sponsored HYDRA 10 experiment that saw Théo Mavrostomos dry-dive in a chamber to 701 meters (71.1 bars) - the world's absolute saturation dive depth record. Though that experiment used "only" 20% H2 (https://www.newscientist.com/article/mg13618493-000-technolo...).