Where should I begin?
I recently moved from PA to southern NJ and with little luck in finding good mud in PA, I decided to look in NJ!
I looked at topographic maps and the locations of state parks, and I found a sinking location that had potential: Parvin State Park in southern New Jersey, which according to the map, half of which is a forested swamp. I was right! The park was full of mud and boy... was it deep(isn)!
Luckily for me, the spot I sank into today was fairly secluded because it's located at a part of a disused trail. The mud was waist-to-chest deep and was slightly watery, but still thick enough to enjoy. However, since it was in a forest, the spot was full of twigs, underwater branches, etc, thus making a deep spot a challenge. I was unable to lay back in the mud because of this. :/. I intend to go back next weekend to explore the park more
This brings me to my next topic...
. Does anyone in PA or NJ know of any deep mud that's near-bottomless (or not in a forest 
)?
I also used a topographic map overlay for Google Earth to find marshes/swamps/etc. Personally, I'm ruling out quarries for legal and safety reasons, but hopefully with further searching, I'll find mud that's relatively free of debris in it. 
... probably because it wasn't smooth or creamy mud 
. It was gritty and full of twigs... even found the odd plastic cup (I hate people who litter at parks!) in the water. Here's hoping to finding something deeper and smoother!