 As deep water surfaces, the pressure drops and a vast amount of gas comes out of solution.Under these circumstances carbon dioxide is hazardous because it is heavier than air and displaces it, so it may flow down a river valley to human settlements and cause mass asphyxiation.The material at the bottom of a lake, or "lake bed", may be composed of a wide variety of inorganics, such as silt or sand, and organic material, such as decaying plant or animal matter.