Laboratory data on scalable Technology Readiness Level (TRL) 4 and TRL 5 prototypes for subsea electrolytic hydrogen production are presented. Calculations based on linear extrapolation of the data describe properties and productivity of gigawatt-scale subsea electrolytic hydrogen production powered by gigawatt-scale offshore wind farms. GE Renewable Energy announced a 14 MW offshore wind turbine. For wind conditions at a typical German North Sea site, GE quotes an annual electrical energy output of up to 74 GWh. Electrolysis is a self-pressurizing process. A GTA TRL 4 electrolyzer was pressurized in the laboratory to 17.2 bar (250 psi) and operated at a cell voltage of 2 V, corresponding to an energy requirement of 55 kWh/kgH2. A 10 x 10 array, 1.4 GW wind farm, will produce ~1.3 x 10⁹ kg H2 per year. Each 14 MW wind turbine powers a 14 MW GTA subsea electrolyzer system that is anchored beneath it at the sea floor. The electrolyzers form a resilient, EMP-hardened grid via connection to a network of subsea hydrogen pipelines. The grid is resilient because loss of one turbine/electrolyzer pair does not affect hydrogen production by any other pair. Electrical conductivity of seawater restricts penetration of EMP pulse radiation to no more than a few centimeters. The autonomous subsea electrolyzers are digitally controlled and do not require scheduled maintenance because (1) they contain no moving parts; (2) polyethylene has virtually 100% stability in seawater (ISO 22404, 2019); (3) potassium hydroxide is a thermal stabilizer for polyethylene (N. P. Cheremisinoff, 1997); (4) nickel electrode alkaline electrolyzers rarely need servicing (B. Kroposki et al., 2006). Hydrogen is safely produced and stored at the seafloor, away from population centers, and in the absence of combustible oxygen. Rectifying electrical output from the turbines and immediately performing subsea pressure-balanced electrolysis to produce hydrogen confer multiple techno-economic benefits: (1) Large power transformers, offshore platforms and inter-array and export high-voltage cables, all high capex and opex budget items, are eliminated. (2) Transport of energy in the form of hydrogen in pipelines is at least eight times less expensive than transport as electricity in metal cables (McKinsey & Co, 2021). (3) Cold deep-water hydrostatic pressure is leveraged for “free” cooling and first-stage pressurization without mechanical compressors. (4) Large-scale hydrogen production and storage at the sea floor are fail-safe with respect to subfreezing weather during periods of unscheduled power shutdown. The most common cause of unscheduled shutdown is unfavorable weather preventing power generation through the wind turbine generators. (5) Subsea electrolyzers are shielded from lighting strikes, inclement weather, ice floes and the destructive power of high waves. (6) Subsea electrolysis is the quickest and least risky approach to large-scale green hydrogen production powered by offshore wind and ocean energies because each unit operation is already performed by the offshore gas and oil industry including subsea electrification, subsea reverse osmosis of seawater and subsea electrolysis.