This may sound counterintuitive, but perhaps the best way to warn future generations of the danger of the area would be to make the area MORE dangerous. That is

This may sound counterintuitive, but perhaps the best way to warn future generations of the danger of the area would be to make the area MORE dangerous. That is, install a highly radioactive cube of something that will kill -- very quickly -- anyone who comes within a mile or two of the site. People will learn to avoid the area very quickly.