Nays Baghai gave a very specific rule to all his collaborators - "Don't pay attention to the way other documentaries have tackled the stories and concepts of diving. This is not a 60 Minutes journalistic piece or a Red Bull adrenaline fest. Imagine we're making Blade Runner (1982) or Inception (2010). Let's give this a more cinematic feel and pay homage to the classics we all grew up with." He also cited The Godfather Part II (1974) as a major influence on the approach to the character-driven style of storytelling and unorthodox structure of the film.
Nays Baghai and producer Kearie Peak took a very deliberate approach to the overall production schedule. The master interview for Jill Heinerth was filmed four months ahead of principal photography to get the psychologically demanding aspects of the interview completed first. That way, the remaining shoots would feel comparatively easier from a performance perspective. It also fostered a strong trust between director, producer and subject, and also provided evidence to financiers that the filmmakers were delivering. Lastly, it also allowed the edit to take shape and for the direction of the story to be clarified before the intensive period of filming began.
Before filming began, Nays Baghai set a new precedent amongst film directors by becoming certified to cave dive on a rebreather. Although he was already an experienced diver with nearly 200 logged dives and a strong degree of comfort on his rebreather, he knew the risks of cave diving necessitated taking a course to ensure filming underwater would proceed safely. The course took place in the frigid waters of Mount Gambier, Australia in September 2022. Despite the grueling nature of the tests, he persisted and passed the course - which enabled him to dive far beyond where most recreational divers (let alone film directors) would be able to go.
Nays Baghai cited the opening sequence as the single most difficult scene to edit from a storytelling perspective. According to Baghai, the sequence went through dozens and dozens of drafts, and originally contained footage shot in Mexico's Kan Luum lagoon set to temp music from Blade Runner 2049 (2017). But the eerie atmosphere, cluttered narration and complex structure forced the core team decided to rewrite the opening and "go back to the basics". Eventually, he found unused footage from the shoot in Mexico whilst separately rearranging archival footage and the narration, and after finessing it with Kearie Peak and Mehrdad Baghai, it finally elicited the result they had been waiting for.
To help capture a variety of camera angles (and also make filming more efficient), cinematographers Janne Suhonen and Richard Stevenson shot with two different lenses - a wide angle 16-35mm for the wide shots and a telephoto 24-70mm for the closeups. All the underwater sequences in Mexico were exclusively filmed with the Sony A7SIII to capitalise on its low light performance, long battery life, small size and overall ease of use.