100yo? they said four decades. Thats 40 years.
Considering the quantity said to have been stashed, that might well leave a fair few doses, even after this time. If it's been buried, it'll keep it out of the light. And considering your average LSD chemist knows their shit, they will be well aware of it's intolerance for light, oxidation, or being kept wet. So if they were going to bury a stash, you'd think that, if they were bright and talented enough to have made acid in the first place, they'd not be so stupid as to not stash it in containers filled with dry nitrogen or argon, desiccant packs, and to keep anything in a jar or other container with a metal lid wrapped in plastic, the metal covered in epoxy, or whatever such measures, with the acid in a separate, inert gas-purged container within the outer container, and that thoroughly insulated from the environment.
Cold, dry, anoxic and out of the light, that's probably quite favourable for acid to survive for a fair time.
And hey, at least the filth aren't going to find it themselves, and will have their little curly pink piggy tails tied up patrolling until their trotters ache.
Whilst the dealers, consumers and cooks have a party, knowing all the little piggies have gone wee-wee-wee all the way to market.
Hehe, and here's hoping now we know where they are, that a few urban snipers will come out of the woodworks and start popping piggy heads off fat piggy necks on a Yautja-style trophy hunt.