World’s deepest Marathon
Sound Design and Mix by JETSONIX
The Project
Set 1,120 metres below sea level in Sweden’s Garpenberg mine, The World’s Deepest Marathon follows 55 runners attempting to complete a full 42.2-kilometre marathon in one of the most extreme environments on Earth.
The challenge combined relentless heat, high humidity, darkness, narrow tunnels and a physically demanding 2.4-kilometre loop. Alongside the runners, the film explores the technology, people and systems shaping the future of modern mining.
The result is a character-led documentary about endurance, human potential and the willingness to redefine what is possible.
The Sound Challenge
The mine needed to feel vast, physical and alive without overwhelming the runners’ stories.
The mix balanced authentic location recordings with breathing, footsteps, movement, voices and machinery, moving between:
The mine’s scale and pressure
The runners’ physical strain
Moments of calm and tension
The emotional weight of personal testimony
The energy of the race’s key moments
The challenge was knowing when to build the sound and when to strip it back.
The Approach
Using the production recordings, music and designed effects, we shaped the film around several deliberate moves:
Added subtle wind to keep the underground environment present
Protected dialogue moments where music was masking key lines
Faded music away during the descent to foreground voices and mechanical sound
Reduced the score to create a more immediate, diegetic feel
Stripped back music and effects beneath the film’s more poignant voiceover
Reduced “epic” musical weight when it competed with emotional dialogue
The sound design grew with the film, but not in a straight line. Some moments needed scale. Others needed restraint.
The most effective choice was often subtraction: removing music, reducing effects and creating enough space for a voice, a breath or a footstep to carry the scene.
The aim was not to make the mine sound artificially dramatic. It was to make the real experience feel immediate, controlled and emotionally precise.
The Result
A cinematic documentary mix that balances industrial scale with human intimacy.
The final sound gives the mine weight while keeping the runners and their stories at the centre. It supports the film’s biggest moments, but also knows when to disappear.
The creative team responded:
“It sounds amazing! You’ve really nailed it.”
And:
“Fantastic work. It’s sounding 👌”
That is the job: not adding noise for the sake of noise. Building the right emotional pressure, then getting out of the way when the story needs room to breathe.
CREDITS
Director & Production Co: Sam Strong | Chrome Productions
Exec Producers & ECD: Joel Mishcon, Mark Wilson, Paul Gurney, Stephen Parker
Producers: Melly Cook (Global Head of Prod), Josh Hay
DOP & Camera: Neil Gordon, Matthew Sellers, Gustav Sandegård, Patrik Nilsson
Post & Tech: Ben Turze, Joel Ogden, Andy Cardy (Editor), Vic Parker (Colourist)
BecomingX & Talent: Bear Grylls, Paul Gurney, Callum Knight (SEApeekay)
Sound, Gaffer & Stills: Julian Willson, Sebastian Hinds, Samuel McElwee, Andrew Goss