On: Alien life form ‘is here on Earth’

[wooster]

  whose inky waters are death to mortals

And yet you can swim in the lake. Snorkeling there is supposed to be quite the experience.

If you want a really scary lake, try Lake Karachay:

  According to a report by the Washington, D.C.-based
  Worldwatch Institute on nuclear waste, Karachay is 
  the most polluted spot on Earth.

http://en.wikipedia.org/wiki/Lake_Karachay

Also: http://www.damninteresting.com/in-soviet-russia-lake-contami...