quicksand

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Sand that is mixed with water in a collected mass and yields easily to pressure so that objects on its surface tend to sink and become engulfed.
• noun A place or situation into which entry can be swift and sudden but from which extrication can be difficult or impossible.

from The Century Dictionary.

• noun A movable sand-bank in a sea, lake, or river; a large mass of loose or moving sand mixed with water formed on many sea-coasts, at the mouths and in the channels of rivers, etc., sometimes dangerous to vessels, and especially to travelers.
• To catch in a quicksand.
• To cover with quicksands: as, a quicksanded coast.

from the GNU version of the Collaborative International Dictionary of English.

• noun Sand easily moved or readily yielding to pressure; especially, a deep mass of loose or moving sand mixed with water, sometimes found at the mouth of a river or along some coasts, and very dangerous, from the difficulty of extricating a person who begins sinking into it.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Wet sand that things readily sink in, often found near rivers or coasts
• noun Anything that pulls one down or buries one metaphorically

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a treacherous situation that tends to entrap and destroy
• noun a pit filled with loose wet sand into which objects are sucked down