The Collaborative International Dictionary of English v.0.48:

Serbonian \Ser*bo"ni*an\, a.
   Relating to the lake of Serbonis in Egypt, which by reason of
   the sand blowing into it had a deceptive appearance of being
   solid land, but was a bog.
   [1913 Webster]

         A gulf profound as that Serbonian bog . . .
         Where armies whole have sunk.            --Milton.
   [1913 Webster]