How do you leave a warning that lasts as long as nuclear waste?

In January 1997, the crew of a fishing vessel in the Baltic Sea found something unusual in their nets: a greasy yellowish-brown lump of clay-like material. They pulled it out, placed it on deck and returned to processing their catch. The next day, the crew fell ill with serious skin burns. Four were hospitalized. The greasy lump was a substance called yperite, better known as sulfur mustard or mustard gas, solidified by the temperature on the sea bed.
Read more: https://arstechnica.com/science/2019/09/how-do-you-leave-a-warning-that-lasts-as-long-as-nuclear-waste/?source=Snapzu