Earlier quoted context omitted.
Options: 1. So you start digging and find huge concrete cylinders with clearly marked "radioactive" warning signs. Hilarity ensues, people sue each other, and cylinders get moved somewhere else. 2. You just build a house on the ground (without digging). Nothing bad happens. 3. Civilization collapses, nobody cares about the abandoned casks.
>1. So you start digging and find huge concrete cylinders with clearly marked "radioactive" warning signs. Hilarity ensues, people sue each other, and cylinders get moved somewhere else. The radioactive symbol (trefoil) was invented in 1946. In 2007 a new supplementary symbol was created. Sure it still includes a trefoil, but it's smaller. Now just imagine the symbols we used today, what we think is obvious might not…
Sure, every random Zakjsnd, Joe and Yelaldto won't know English, but archeologists certainly will. And they'll be able to read it.
But even if we suppose that a nearby supernova explosion resulted in a burst of magic rays that somehow wiped all memories, people will either:
1. Know what radiation is, and they'll test casks for it.
2. They won't be doing digging down for at least tens of meters.
You really need a very unlikely confluence of factors: a very developed, but still pre-technological civilization, that is somehow interested in digging way down (why?) in that particular spot.