Breathing gas for the diver is piped down from the surface, through a long, flexible hose, bundled with other services and called an diver's umbilical. In addition to breathing gas, the umbilical will have additional hoses and cables for such things as communications equipment, a pneumofathometer for measuring depth, or hot water should the diver be using a hot water suit. The umbilical must be strong enough to support the diver's weight, with a significant safety margin, because it may be used by surface personnel to pull the diver out of the water. The diver's breathing gas is pumped down from either high pressure tanks or through a gas compressor.