Diatoms are microscopic algae with hard, siliceous cell walls (frustules) that are abundant in most types of water bodies (freshwater, brackish, and marine). In cases of death by drowning, if the person was alive when entering the water, diatoms are inhaled into the lungs and can enter the bloodstream, circulating to various organs like bone marrow, liver, kidney, and brain. Their presence in distant organs is a strong indicator of vital drowning, as opposed to post-mortem submersion.