Failure to control depth due to insufficient buoyancy can also lead to scuba accidents. It is less of a problem with surface supplied diving as the depth that the diver can sink to is limited by the umbilical length, and a sudden or rapid descent can often be quickly stopped by the tender. In early diving using copper helmets and a limited flow air supply, a sudden rapid descent could lead to severe helmet squeeze, but this is prevented by demand supplied gas, and neck dams on later helmets, which allow water to flood the helmet until the gas supply catches up with the compression. Surface supply ensures that the gas supply will not run out suddenly due to an unexpected high demand, which can deplete scuba supply to the extent that there may not be enough left to surface according to plan. Any sudden increase in depth can also cause barotrauma of the ears and sinuses if the diver cannot equalise fast enough.