Effects of Breathing High Pressure Gas: In the case of a drysuit, it does exactly what the name implies: keeps a diver dry. The suit is waterproof and sealed so that frigid water cannot penetrate the suit. Drysuit undergarments are usually worn under a drysuit to keep a layer of air inside the suit for better thermal insulation. Some divers carry an extra gas bottle dedicated to filling the dry suit. Usually this bottle contains argon gas, because of its better insulation as compared with air. Dry suits should not be inflated with gases containing helium as it is a good thermal conductor. Drysuits fall into two main categories: neoprene and membrane; both systems have their good and bad points but generally their thermal properties can be reduced to: Membrane or Shell drysuits: usually a trilaminate construction; owing to the thinness of the material (around 1 mm), these require an undersuit, usually of high insulation value if diving in cooler water. Neoprene drysuits: a similar construction to wetsuits; these are often considerably thicker (7-8 mm) and have sufficient insulation to allow a lighter-weight undersuit (or none at all); however on deeper dives the neoprene can compress to as little as 2 mm thus losing a proportion of its insulation. Compressed or crushed neoprene may also be used (where the neoprene is pre-compressed to 2-3 mm) which avoids the variation of insulating properties with depth. These drysuits function more like a membrane suit. Injuries due to contact with the solid surroundings: Diving suits also help prevent the diver's skin being damaged by rough or sharp underwater objects, marine animals, coral, or metal debris commonly found on shipwrecks. Hazards of marine animals: See also: The specific diving environment