Originally posted by jonnin:

Muck is sneaky and tends to leave a LOT of tiny little bits that you can hit with some wiggling around. Each one has a shot at iron, no matter how tiny. Make sure you clear these if you did not.

I've gone into Sunken Crypts my partners have previously mined and come out with an extra 10-20 iron. Some of those little remnants of Muddy Scrap Piles are very hard to see, and very easy to miss, one definitely has to meticulously inspect the ground, especially where the walls meet the ground in the corridors that were previously filled with scrap piles. The Dverger Circlet is a very handy item to wear when exploring crypts & burial mounds, it sure beats carrying a torch.

Wearing the Wishbone in the Swamp (rewarded after killing Bonemass) will detect Muddy Scrap Piles, which can be found outside of crypts on the ground. Most of the ground based sources of Muddy Scrap Piles I have found have been along the outside edges of swamps, much of it where the swamp borders another biome, especially The Black Forest. IMO that source of scrap iron is not worth the effort as it rarely yields more than 1 scrap iron.