The Hundred-Century Challenge

From tomorrow's weather to next year's salmon runs, what will happen in the future commands a lot of scientific attention. Recently, a group composed predominantly of scientists was asked to devise a warning sign that will last for ten thousand years, and that will be understandable as a warning of extreme danger to anyone who sees it ten thousand years from now.

The warning sign is needed for the first nuclear waste repository, where buried plutonium-contaminated materials are to be entombed for millennia far beneath New Mexico. But, to quote Alan Burdick, a science editor who wrote of the problem in the August 1992 issue of Harper's Magazine, "Every tomb needs a tombstone." This tombstone must be special indeed.

The team composition reflected the complex task: psychologist, linguist, archaeologist, anthropologists; materials scientists and an architect; two astronomers who had worked in the National Aeronautics and Space Agency's quest for ways to communicate with extraterrestrial intelligences, should we ever encounter any; and an artist who had worked on the diagrams sent into space aboard Voyager.

Going by history, their task looks tough. Consider, for example, the inscriptions on the Egyptian pyramids. Their hieroglyphic language was once understood by all literate residents of the Nile Valley. Now only a few specialists can decipher the symbols, but the pyramids aren't yet five thousand years old.

Stonehenge is a tough, simple structure but archaeologists fight about its exact purpose and meaning. It is only 3500 or so years old.

The warning-sign team split over how best to communicate danger to the future. Half the group advocated a sign incorporating various messages from terse commands ("Danger: Poisonous Radioactive Waste Buried Here. Do Not Dig Until A.D. 12,000") to detailed lists of the repository's contents. The messages, they thought, should be repeated in many languages and different alphabets: English, Spanish, Chinese, Russian, French, Arabic, and the local Mescalero Apache. This group wanted only two pictorial symbols, a stylized depiction of a screaming face and the trefoil sign for radioactivity---a symbol they believed humankind would have no chance to forget during the coming hundred centuries. The other half of the group doubted any language or alphabet would last. Its members chose pictograms, devising a kind of wordless cartoon strip of fleeing stick figures.

How to write the sign was less debated than how to build the signpost. The structure housing the message must be massive and tough enough to withstand not only natural erosion but human vandalism. (The limestone blocks covering the Sphinx have eroded somewhat during a few thousand years of wind and rain, but the great statue's nose vanished quickly when Napoleon's troops used it for target practice.)

The group members agreed that nothing anyone would want to carry away, in fact nothing anyone could carry away, should be used in its construction. Thus, one suggestion was for a square-mile field of massive boulders laid out in a tight grid pattern: the Forbidding Blocks. Another, the Rubble Landscape, was a high pile of stony detritus designed to resist drilling and digging machinery.

The construct should look and feel unfriendly, such as in the Landscape of Thorns: a square mile of 80-foot-high black basalt spikes jutting from the ground at all angles. The Spike Field set the same basalt spikes into a regular, uniform pattern; the Black Hole rolled them flat, or used black concrete to produce the same effect. A favorite was the Menacing Earthworks, with 50-foot-high berms like jagged lightning bolts surrounding a walk-on map of the world's radioactive waste storage depots.

One of these suggestions may be built some day, but no one can predict how well it will work. On the evidence, probably not very well: In 1961, an underground nuclear test took place ten miles from the plutonium-waste repository. After much searching, author Burdick found the warning plaque marking the spot. The granite block was overgrown by mesquite bushes; the iron letters had rusted into unreadability.