High-level nuclear waste emits radiation for tens of thousands of years.  So, for about 50 years now scientists across the globe have been seeking the safest way to permanently dispose of it.  They've considered a lot of ideas -- like shooting it into outer space, or embedding it in polar ice caps.  Finally they resolved to put it deep underground in facilities called "geologic repositories".  That's what's now being plotted for Yucca Mountain in Nevada.  But what will Nuclear Regulatory Commission put on the shingle hanging out front at Yucca?  How will these sites be visually marked for generations in the distant future -- generations that by that point may have survived all kinds of changes we can't picture -- wars, revolutions, natural disasters?  There's no real solution yet.  The challenge is sort of like branding danger for all eternity, and it's so complex it may take us nearly that long just to sort it out.