Badwater Basin. 
Badwater Basin, more than 280 feet below sea level, is the lowest point in North America.  The salt flats, which are formed by drainage from the surrounding mountains and evaporation due to the high temperatures, cover nearly 200 square miles.  Underneath are layers of mud, and some interesting salt-loving microbes can be found there.  The expanse of salt flats is so impressive that one tends to overlook the springs at the edge, which are home to the Badwater snail (Assiminea infirma), a small mollusc.  In ancient times (late Pleistocene) this area was a giant lake, Lake Manly, which was about 90 miles long and up to 600 feet deep.  Badwater is also known as the starting point for the Badwater 135, a 135-mile run described as "the most demanding and extreme running race offered anywhere on the planet (+)."
Note the sea level sign. 

Microbes
Brian Adam Schubert.  "Long-term survival of prokaryotes in subsurface halite, Death Valley, California."   ProQuest LLC, 2009 [2004 paper for dissertation].

Tim K. Lowenstein, Brian A. Schubert and Michael N. Timofeeff.  "Microbial communities in fluid inclusions and long-term survival in halite."  GSA Today, Volume 21 Issue 1, Jan. 2011.

National Science Foundation.  "Badwater Basin: Death Valley Microbe Thrives There."  Dec. 22, 2011 [press release].
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