a deadly poisonous lake... Mono Lake, in California’s Yosemite National Park... one of the highest natural concentrations of arsenic
I thought forbidden lakes whose inky waters are death to mortals were the stuff of fantasy novels. (In fact, any Terry Brooks fans think this was the inspiration for the Hadeshorn?)
(After a bit of further reading, it seems the article overplayed the "deadly" aspect, since fish live in the lake, birds eat there, and humans have dived in it: http://en.wikipedia.org/wiki/Mono_lake)