I'd start by taking the deck floorboards off and let the sun get to the soil as well as to the undersides of the deck boards. It sounds like a mix of mold and mildew and the mud comes from rain onto compacted soil. You might call your termite pest control company for some advice also.

4 feet of crawl space under the deck should be sufficient for air circulation once the area under has dried and is treated for the gunk growing on the surface. See if you can figure out what is blocking the drainage from this area and correct it. Does the house have gutters? Are the downspouts into piped drains or just splash blocks by the foundation?

Have it tested by the ag extension office. I also agree wholeheartedly that it needs some type of air circulation. Where does your sewer line run?

I would guess a leak in a pipe somewhere. Please let us know when you find out the cause of the problem.

What part of the country do you live in? I live in Washington State and everything here molds unless the rust has killed off the mold and taken over.

Also, do you have a little dial on your water meter, that twirls when the water is running in your house or yard? If so, turn off all water indoors and out, and then look at it. If the little dial is still twirling, you've got a water leak and need to get it fixed.

Missouri. No water leak. Sewer line runs from the front of the house out to the main. This area is in he back on the south side of the house. Guttered but the downspouts had only been running about 2' away from the house. Last fall I fixed it so that they are all running into water barrels or a good 6 feet from the house. A couple of months ago I dug a deep trench away from this area going down hill. Yesterday, I dug down back about a foot from where the trench started and it is still definitely the same wet, smelly, compacted soil that it was. I am taking it to the extension office on Monday. The deck s going to come down in the relatively near future. It really can't be repaired. When it does I will be able to unearth what I can. The house itself has a sub-pump in the basement. The sub jump had a lot of debris around it and had not been running for about 6 months. It was replaced last fall. I had a lot of mold in the basement which has been rectified. If it were wood, drywall etc it was tore out. The concrete was treated professionally. Thank you for every bodies input. It is appreciated.

Thanks for the update!