These incredibly rare mushrooms are one of the only species that still exist to this day from the pre-supernova era of The Expanse.
As shown in their name, they appear once every ten thousand years in massive clusters that stick around for about a year before disappearing.
Trying to preserve these mushrooms have proven to be incredibly difficult. Modern special canisters can preserve them for about 30 years, but it's not long enough to bring out the true benefits of the thing.
The only way to preserve them long term is through extremely old ancient canisters, and very few of those are still around to this day.
All our attempts at reverse engineering these have failed. There was something that the civilization before us knew that we did not.