The documentary 'Into Eternity' chronicles the building of a nuclear waste storage facility in Finland. Designed as a "permanent solution" for radioactive material - which will remain hazardous for at least 100,000 years - the huge underground site is hewn out of solid rock. As such, the site at Onkalo (meaning 'hiding place' in Finnish) is built to last longer than any manmade structure ever has.
It is a mindboggling timescale, and the documentary takes an appropriately philosophical angle, musing about civilization, myth and time - but with enough humor to show that the stoic Finnish scientists can't quite wrap their head around it either. As one of the project directors sighs, "It is quite possible we will not be understood by the future".
Another question raised by the project is whether or not the site should be marked for future generations. Here, too, the enormous timeframe poses fascinating and rather absurd problems. Some people say the site is best left to be forgotten and shouldn't be marked at all, as any sort of warning would only make future discoverers of the site more curious; while others maintain that markers are a moral responsibility.
This leads to the further question of how to communicate danger to people 10,000 years or more into the future, when communication itself may have changed beyond recognition. Languages, knowledge and symbols all change - even apart from the fact that few 'media' would be durable enough for the warning message to survive. From the past, famous examples like the Rosetta Stone, Stonehenge or the Pyramids come to mind, but these are all less than 5,000 years old.
In the 1990s this problem was studied in the U.S. for a similar nuclear waste site, and a lengthy report was published containing a whole range of design concepts, ranging from basic cartoon strips like 'Skull & Crossbones' to fullblown architectural landscapes with names like 'Landscape of Thorns' and 'Menacing Earthworks'. It makes for a fascinating read, this report, fully titled 'Expert Judgement on Markers to Deter Inadvertent Human Intrusion into the Waste Isolation Pilot Plant' (excerpts or the full report in PDF).
However, as 'Into Eternity' points out as well, it would be rather sad if the longest lasting remnant of our civilization would be a warning sign for a lethal garbage dump...
Update: As a witty friend remarked, the 'Skull and Crossbones' image can also be interpreted: 'Warning: contains heavy-metal pyjamas'.