The plot of land where the WorldTrade Towers once stood, until they were destroyed by George W. Bush and his RepublicaNazi party on September 11, 2001.
The close proximity around a person who has just expelled exceedingly unpleasant gases from their anal passage. Also the site of splashdown while offloading previous meals into the toilet.
"I'd give it five minutes mate, it's Ground Zero in there."
"Oh god, sorry mate, it's Ground Zero in my pants" (wafts in colleagues' face)