27/02/2026
History Friday: 701 Metres - COMEX Hydra X
By the early 1990s, commercial saturation diving had pushed heliox close to its limits. High Pressure Nervous Syndrome (HPNS) was the barrier. The question was simple: could hydrogen take divers deeper?
That question led to Hydra X, the final and most ambitious programme in the COMEX.SAS hydrogen series.
Hydrogen had already been trialled in Hydra VIII and IX. Less narcotic than nitrogen and capable of lowering gas density, it showed promise in reducing HPNS tremor and improving breathing resistance at depth. But long-duration saturation at extreme pressure was still largely unproven.
In November 1992, Serge Icart, Régis Peilho and Théo Mavrostomos entered the COMEX hyperbaric complex in Marseille. After baseline testing at 10 metres, they were progressively compressed to 625 metres (2,050 ft), later transferring to high-pressure spheres at 675 metres (2,214 ft). Physiological monitoring was continuous: EEGs, cardiovascular tracking, tremor analysis and cognitive testing formed part of the programme.
On 20 November 1992, Mavrostomos descended alone to 701 metres (2,300 ft) for approximately three hours, conducting calibrated work simulations designed to replicate underwater tasks. It remains the deepest simulated dive ever performed.
Decompression began the following day and lasted 24 days, with staged stops and in-chamber work periods. In total, the divers spent 43 days under pressure.
Hydreliox demonstrated improved control of HPNS compared with heliox alone. Tremor was significantly reduced, functional performance remained high, and decompression monitoring showed no significant bubble formation. However, hydrogen was not a complete solution, subtle neurological effects persisted, and the operational complexity of managing a flammable gas mixture at extreme pressure presented major offshore challenges.
By the mid-1990s, advancing ROV capability and shifting commercial priorities reduced the need for ultra-deep manned intervention. Hydra X became the closing chapter of the hydrogen frontier.
For some great archival imagery dedicated to Hydra X, this gallery is worth exploring:
https://lnkd.in/egtd44NM
It wasn’t simply about reaching 701 metres. It was about validating physiology, gas science and life-support engineering at the very edge of human capability.
📸 Image credit: COMEX Archives / Alain Tocco
