Coal forms from the accumulation of plant debris, usually in a swamp environment. When plant debris dies and falls into the swamp the standing water of the swamp protects it from decay. Swamp waters are usually deficient in oxygen, which would react with the plant debris and cause it to decay.
Porokeratosis is generally considered to be a benign condition though malignant transformation has been seen in all but the punctate variety, necessitating regular followup of patients with porokeratosis (Maubec et al., 2005; Sasson & Krain, 1996).
Ovary Disease lists cystadenomas as one type of non-cancerous hypoechoic cyst that is filled with mucous. Another example of a benign hypoechoic cyst is an endometrioma, sometimes called a chocolate cyst.. An endometrioma is a blood-filled cyst formed from endometrial tissue and found inside the ovaries.