Extremely deep bounce dives: planning and physiological challenges based on the experiences of a sample of French-speaking technical divers.

Introduction: Extreme deep technical diving presents significant physiological challenges. While procedures often blend elements from both recreational and commercial diving, many remain empirical and unvalidated for this purpose. The rise of closed-circuit rebreathers has reduced gas cost and logistical barriers, enabling more divers to reach unprecedented depths. This study, based on the experience of deep divers, explores the limits of extreme-depth diving and the strategies developed to overcome them. Methods: Eight rebreather divers (one female, seven males) with experience beyond 200 metres depth were interviewed regarding their preparation, planning, and execution of such dives. The dive profiles of their deepest dives were analysed. Results: All were highly experienced technical divers. The median maximal depth was 227 [209-302] metres, with a median total dive time of 290 [271-395] minutes. The gas density of the trimix mixture, oxygen exposure, and ascent rate consistently exceeded current recommendations. High pressure nervous syndrome did not appear to be a major limiting factor, whereas decompression posed greater challenges. Three divers experienced decompression sickness following their deepest dives, highlighting the uncertainty around decompression procedures. Conclusions: These dives require rigorous preparation, robust support systems, equipment modifications, and perfect skills to reduce risks, which remain excessively high. Data are lacking to validate current practices. Decompression procedures must be adapted for these demanding mixed-gas dives, which are inevitably prolonged. A dry underwater habitat could improve decompression tolerance. The role of hydrogen as a breathing gas remains uncertain and still needs to be clarified, but some consider it a promising avenue for further exploration.