This helmet was designed by Commander S.S. Hall, R.N. of the marine service and Staff Surgeon Oswald Rees of HMS Mercury and manufactured by Siebe Gorman & Co., Ltd. It's purpose was to provide a means of escape from a submerged submarine. It was designed for diving in shallow depths with a maximum use time of forty-five to one-hundred-twenty minutes. It is light weight and can be donned by one person, without assistance in thirty seconds or less. It could also be used as a smoke hood.

The apparatus consisted of the helmet, which slopes away to fit the shoulders, conical rings to fit over the shoulders, which then continued into a short waterproof jacket which belted at the waist. The original apparatus had a flexible hood, but that was later abandoned in favor of a metal helmet which protected the wearer's head. The helmet has an air-tight window, and the sleeves of the jacket were provided with rubber tightly fitted round the wrists, like a diving suit, while the belt around the waist held the loose jacket in place. The jacket acted as a life-preserver to keep the wearer afloat.

In the front of the jacket is a pocket containing an "oxylithe" (Kalium-Natrium-Superoxid aka sodium peroxide) purifier and oxygen generator canister, which gave off oxygen when breathed on and absorbed carbon dioxide. Eliminating the need to carry compressed oxygen.

The diver breathes via a tube through the purifier, which is connected by another tube with the air-space inside the helmet. The purifier is situated so that it cannot be blocked.

Despite its workable design the Hall-Rees diving dress was phased out because carrying one for each crew member in a cramped submarine simply took up too much room.