I agree with Maria, the nicest spots are the ones you find unaware sometimes. There was an interesting bog I would go to often the last 4-5 years but each year the bog became covered over with foliage and tall grasses until the mud was longer noticeable,it does happen a lot as temperatures change from year to year,certain spots here have dried over with draught like temperatures.
The best locations always seem to be around swamps and marshes where there is a lot of humidity and one area I was surprised was near this marshy meadow that was moss covered and hiding a good 3-4 feet of black mud,sometimes the surface seems solid and really isn't, the squelching noises were interesting,very difficult to extricate yourself.
The one thing people should think when they are wearing wellies or waders is buy the cheapest ones that you wont mind losing or find an older pair at a thrift store if you can

I have lost a few boots and sometimes when they are glued in they are not easy to dig out.