HYDRAV was performed in COMEX Marseille hyperbaric Research Center in May and June 1985.During this Experiment, 6 divers lived at a simulated depth of 450 meters in a hyperbaric atmosphere composed of hydrogen, helium and oxygen (hydreliox). Tradition deep diving with helium is limited by 2 Factor: – High Pressure Nervous syndrome (HENS) that causes Motricity disorders beyond 300 meters. – Gas density that impairs pulmonary ventilation. Hydrogen should extend these limits thanks to its Narcotic potency that has an antagonistic effect on the HPNS syndrome and thanks to its light molecular mass. HYDRAV, first hydrogen experimental saturation dive, was successfully performed. All the equipment and techniques especially developed to use hydrogen have proved entirely satisfactory. From the biomedical standpoint the improvement with reference to Heliox is outstanding: – No High Pressure nervous Syndrome, – Easier breathing. The very promising results of HYDRAV will hopefully lead to quick offshore industrial applications. Hydrogen will enable divers to work in water depths Ranging from 400 to 600 meter and certainly beyond. INTRODUCTION: For more than 20 years now, all deep commercial Dives have been carried out using breathing mixtures Made up essentially of helium and oxygen. Helium has allowed the professional diver to work on the continental shelf area down to depths of 300 meters. In this range, equipments and methods are now relatively well established. However, in spite of its remarkable qualities helium alone cannot be the vector of oxygen in breathing mixtures beyond a certain depth. Indeed, deep divers must cope with 2 limiting Factors: – The High Pressure Nervous Syndrome(HPNS), linked to the effects of hydrostatic pressure on the central nervous system. These appear in the 250 /300 meter range and increase with depth, Causing motor disturbance that may seriously Impairs work efficiency. – The density of breathing mixtures, which increases proportionally to pressure, demands a subsequent increase in breathing efforts, which hinders work capacity. The following countermeasure may be proposed: – The effects of pressure can be reduced by adding to the breathing mixture a given amount of narcotic as (helium is not). – Gas density can be reduced by using a vector that has a lower molecular mass than helium. Hydrogen is the only gas that satisfies both these criteria. Because of the progress in the search and exploitation of deep offshore oilfields, C0OMEX decided to launch a new program for developing hydrogen diving. Experiment have been carried out both at sea and at Its research center in Marseille. – Hydra III- June1$&x dives at sea at depths of 75 and 91 meters. – Hydra IV – November 1983: chamber dives at depths of 120,150,183,240and 300 meter. – Hydra V – May and June 1985: chamber saturation Dives at 450 meters. The positive results of these series of experiments lead us to envision a rapid industrialization of deep diving using hydrogen breathing mixtures.