Geology is most probably largely how the sealed claim has been made .. it was open to the world when the limestone was forming, formed (Speleogenesis) by sky water (rain) permeating down and eating out a limestone void, and "sealed" by thick and impermeable layers of clays and loess (windborne sediment) some 2.5 million years ago during the Quaternary.

There are also genetic drift estimates, which put the "last" outside species to enter at about that time (

  One of the most recent animals recorded is the cave's only species of snail, Heleobia dobrogica, which has inhabited the cave for slightly more than 2 million years.

~ https://en.wikipedia.org/wiki/Movile_Cave )

Mentioning the recent Chernobyl accident is weird too. Surely an accident from long ago that is expected to be detectable is far more helpful.

All they proved is that nothing had entered since the Chernobyl accident.

"Combined with the geology and the peculiar chemistry of the cave’s water and air, the evidence pointed to a system that had been closed off since roughly five and a half million years ago."

[dead]

while looking at the wiki (https://en.wikipedia.org/wiki/Movile_Cave), this popped up from 2015 from bbc earth: https://web.archive.org/web/20210531110415/https://www.bbc.c...

That has a lot more to do with whether the cave started out containing anything "more complex than insects." 5 million years isn't, like, an evolve-monkeys-from-scratch number of years. It's arguably an evolve-humans-from-chimps number of years. But if your cave doesn't contain any chimps... or even if it does but they all starve to death in Year 2 out of 5,000,000...

the sun is energy so i’d expect there to be a correlation there.

Yeah it has significant groundwater inflow (like 5L/s). Nothing about this is sealed.

[dead]