Turnagain Arm (Cook Inlet right next to Anchorage) has some history with the stuff, espcially the ones where people did not make it... One poor guy was badly stuck and the helicopter that arrived to pull him out, pulled him in half, instead.

The silt here is made of small flakes that orient themselves as flat little linked bridges and as long as one keeps moving the ground seems very firm and solid. A little agitation however and water works up into the bridges lubricating the little plates and liquifying them. (thixotropic is the term and it is exactly why ketchup needs to be stirred to get it moving, yet flows easily once agitated)

A very sad story happened some years back at the head of Turnagain Arm when a young military couple on 4-wheelers decided to run across the mud flats to go gold panning. In crossing a creek bed below the tide line the wife got stuck and when she got off the machine she REALLY got stuck. The husband went for help and the Girdwood Fire Department showed up as the tide was coming in. They could not free her though the had compressed air to blow into the mud in an attempt to dehydrate it. In the end the tide got too high and the firemen could only leave her with a breathing tube. She did not make it. IIRC she was just 19.

There are other stories, but that should convince anyone to keep off those beaches... A couple weeks ago we had the largest bore tides of the year and there were wind surfers, surfers, kayakers, and more out there riding it...