There seems to be the very literal muck diving, and the less literal one. The more being diving in the definition of muck and sort of murky water. The second, less literal version, uses the word muck to describe the sediment and debry that many unique creatures call their home. Man-made "trash" can provide an excellent habitat for creatures that are seeking a refuge form the daily hardships of the coral reefs, or artificial reefs (wrecks). Thats why you will find many juviniilles etc. in muck dives. Mud is a very aerobic substrate, and thus provides a healthy benthic envrionment. In other words: dont dis the muck! I believe when you run into companies organizng "muck dives" or hear about the new kind of diving (like wreck, reef, ice etc.) the second version is the one its refering too. As it becomes more popular, the distinction between macro and muck seems to be bluring a bit. As for now though, you could have a good macro dive on a reef dive, but one could not go for a muck dive on a reef (otherwise it would be a reef dive..duh). Muck diving seems to be a fascinating new "kind" of diving, that perhaps has a bad rep due to its name. But for those people who not only enjoy the large pelagics, the beautiful reefs or the historic wreck, but really enjoy the individual creatures themselves(and could look at them for a looong time, rather than keep moving along a "path") muck diving is a great way to dive!
It was made under a house with a broken sewer line.
Yep... one man's muck is another man's incredible dive... it's that "mind over matter" thing... if you don't mind... it don't matter. 
