Nicolas Massart, AFC, discusses the technical challenges involved in filming "Sous la Seine" by Xavier Gens

Summer 2024. Paris is hosting the world triathlon championships on the Seine for the first time. Sophia, a brilliant scientist, is warned by Mika, a young environmental activist, that there is large shark in the river’s depths. The two women have no other choice than to team up with Adil, the commander of the river police force, to avoid a bloodbath in the heart of the city.

You are not a veteran of underwater filming, so how did you come to work on this film ?

Nicolas Massart : I had met Xavier Gens on the Netflix series "Lupin" and we got along great. When he offered me to work on Sous la Seine, I hesitated at first given the number of challenges it involved… I didn’t know whether I wanted to undertake such a great challenge… But, since Xavier and I got along so well, I decided to say yes. Happily for us, the film’s preproduction was quite delayed due to casting issues. When we began preproduction in September 2022, the film was originally supposed to be filmed two months later, in November. In the end, we began in January 2023. Four months’ time that we really used to our advantage, with the help of the producers who played the game by allowing us, for example, the budget to do three days of tests in the underwater studio that we were going to shoot in. I would really like to thank the core group of executive producer Daniel Delume, assistant director Thomine de Pins, and Arnaud Fouquet of MPC (VFX supervisor) who helped us immensely to clear, test and gradually perfect the method we’d use to make this film.

What were your relations with Netflix like ?

NM : The platform really took a bet on this film. It had a large budget and I think they needed reassurance. To do that, Xavier did a pretty crazy thing, he entirely pre visualized his film over the course of a month in a danse studio in Pantin. Just using his phone, a few boxes and transparent plastic sheets to imitate the water, he asked young actors dressed in t-shirts bearing the names of the characters, to primitively play each scene and each shot of the film. The entire thing was edited together and the result was something like what we see in Michel Gondry’s film Be Kind Rewind. A hilarious and absolutely unique document that I hope will be released as a bonus at some point ! In any case, Netflix really liked it, and this demo absolutely established the rhythm of the future film. It was also very helpful to us in minimizing the number of shots we had to take. Thanks to this pre-visualization, I remember that Xavier was able to be extremely precise in his requests, and we sometimes limited ourself to just shooting a few seconds of each value on some underwater shots, as we’d decided to drop any overly complicated master shots, or ones that would just be useless because of the rhythm of the scene.

Where did you shoot the film ?

NM : The underwater and on-the-water scenes were shot in three places. Studio Lites in Brussels, for everything that takes place underwater and in the catacomb crypt scene. The Cité de la Luz in Alicante, with a large outdoor swimming pool (100 x 80 meters) was where we shot the daytime exterior shots on the ocean and the large triathlon scene, and lastly we shot in Paris for a few days, on location around Notre-Dame Cathedral and the Île-Saint-Louis. I’ll add that it was absolutely impossible to shoot longer in Paris, as it is extraordinarily difficult to get permission to film on the Seine, and you are limited to morning hours between 6-9 am, before there is too much traffic on the river.

So you assembled Parisian backgrounds with foregrounds shot in Spain ?

NM : Yes, we had to use compositing on most of the shots. This included shots with the extras, the sets with foreground shots and water, and an immense, 9-meter-tall green screen covering the entire span of the pool in Alicante.

We shot plates in Paris later on, on the few days we’d been authorized on location. We did a precise inventory of sunlight angles so that we would be able to ensure continuity between the two locations. The weather of course played a few tricks on us, as the sun was absolutely necessary to be able to ensure continuity with the shots taken in Alicante. For the camera movements, I had the great pleasure of working with Benoît Dentan, of XD Motion, who set up the four pylons supporting the CableCam system above the pool in Spain. This is the same system used to film matches at the Stade de France. After one of those matches, we were able to test out the configuration which later on allowed us to move the camera in almost every direction from 10 cm above the water up to 10 meters above it, to shoot the triathlon scene. This setup allowed us to dispense with any crane or traditional grip equipment, which would have been ill-suited to the flexibility this scene, with its many extras, required. With the CableCam, we were able to be very reactive and we could even seek out shots very quickly thanks to the short focal length lenses that Xavier prefers, whether this be a topshot above the swimmers, or a zig-zag shot later on in the middle of them with the shark attacks…

How did you handle compositing with these motion shots ?

NM : For these shots, very precise data regarding the camera position in the space is provided by the CableCam in its metadata. These data are then used by the VFX to shoot the plates in Paris, and then they use them as a background for the movements. I also would like to thank the Dutch "Grip on Water" team (Frerik Scheffer and Hans Broer) who worked with us on this film, and handled all the camera movements on the water, thanks to their special dolly boat outfitted with jets, and not propellors. This was an absolutely wonderful took that allowed them an insane amount of precision in the camera movements on the water, such as the ones you can see in many sequences.

What is striking when you see the film is the many different underwater ambiences, both in terms of textures and colors, in the different scenes…

NM : Yes, portraying the texture of the water was something that was fundamental for me. The true Seine is rather dense and when you dive into it, you don’t see much further than 2 meters deep. For this film, Xavier and I decided to allow ourselves a bit more margin of manoeuvre ! But all the same, we wanted to convey this water that transports so many things, its greenish color, and above all, the presence of the current… For that is the reality of the river in Paris. For that, the test days during preproduction were capital for us, and we tried various means for dirtying the water in the pool. I remember that we first attempted a mixture of brocoli, clay and blades of grass to create the green soup effect on screen. Because we had also planned to use handheld torches with the divers in certain scenes, we did tone down the green a bit, because it wouldn’t go with the red of those lights… We ended up replacing the broccoli with cauliflower ! It was really like being in the kitchen ! To portray the current on the screen we installed jets outside of the frame that created waves in the soup… this did make the actors’ and divers’ job a bit harder.

At what depth did you shoot ?

NM : The pool at Lites is 9 meters deep. But we didn’t generally go that deep. Most of the time, we shot at around 5 meters, at the mid-point of the pool. For the most complicated shots, such as those with the underwater explosions, we set up at 2 meters deep, using a sort of rotating drum that we called the washing machine, which allowed us to have the camera and the actor turn together in a rolling movement around the lens axis. This was a “Grip on Water” creation that was especially designed for this shot that Xavier had imagined. The other originality of Lites Studio is that you can immerse the entire set 1.50 meter under the water. This is what allowed us to shoot the crypt sequence, with a very beautiful set built by Hubert Pouille’s team.

To light this scene, I used very soft levels of lighting for the shadows that I regulated using SkyPanels showering the set from the studio ceiling, and a series of Rosco LED X spots with RGBW effets that allowed us to simulate the reflections of the water - without having to bounce a traditional light source off of water. These are very handy tools whose color can be adjusted on a console and that allows you to immediately have the impression of moving water on each scene.

Let’s talk a bit about the masks, which are one of the key elements in the film’s visuals.

NM : The choice of "full face masks" came to us from the film 47 Meters Down, where we discovered them. You know, as soon as you put a traditional diving mask on someone’s face, you hide the actor’s interpretation of the role immensely. All you have left is the eyes and even those, not so much ! Faced with this dilemma, we sought out these special masks, which are manufactured in very limited quantities by an English company called "Marine Department". Besides the fact that they cover the entire face, they also have glass on each side, which gives the actor a lot of visibility. In fact, these are accessories that were perfected especially for underwater documentaries, giving the divers a much wider visual field. The other reason for this choice was the visual challenge of the catacomb scene. An underwater set that had to be dark because it is underground. In order to make sure that the faces remained visible in wide shots, while maintaining a sufficiently credible and dark environment, we thought that we could install small lights inside the masks…

This is making me think of James Cameron’s Abyss...

NM : Yes, that is one of Xavier’s favorite films, and we did think a lot about it, since at the time they were facing the same issues related to darkness. Because these full face masks are not equipped with lights inside them, we called on Henrik Moseid of Softlights in Paris who is specialized in custom lighting solutions for films. He perfected the integrated lighting solution inside of the mask with, at my request, the possibility of having two color temperatures I could control for different scenes.

The prologue was supposed to be set in the ocean, under the continent of plastic waste, and it used daylight. For the catacomb sequence, I used tungsten lights, which are closer to the underwater red torches that the actors were holding, and which Softlights also designed. Thanks to this combination, I was really able to find the best settings to best capture the faces during my tests, which allowed me to avoid the grey skin tones that you too often see in this type of underwater scene.

Netflix’s battle horse is HDR and 4K… what do you think about this ?

NM : When you’re shooting a film where the water is so present in the visuals, HDR’s relevance is immediately apparent. You immediately notice how the details in the high lights become important. The torches are very brillant… Or just the textures of the bubbles, the incredible shines that give depth to these underwater shots. The Alexa 35, which I was using for the first time, has a lot of latitude and it was very useful. Its color quality, too, which suddenly, even underwater, allowed us to capture details on the lips, the cheeks and the eyes. Even though I know that many cinematographers are doubtful about the interest of HDR, I can tell you that on this film, it did give the visuals a lot of depth and volume.

Was everything really shot underwater ?

NM : Everything except one shot ! The shot where Bérénice Béjo is dragged down to the depths by the shark in the prologue is the only one that was shot in studio outside the water. For this rather complex long take, where we move closer to her as the pressure on her body increases, we used a Motion Control camera operated by Jacques Honvault. During preproduction, we were planning on using this technique for entire sequences in Sous la Seine. But, in the interest of saving time, making communication with the actors easier, and sparing them the discomfort of training to do actual dives. This idea was quickly shelved given the astronomical number of shots involved, as this would have blown through our budget in short order.

On that topic, what was the film’s budget ?

NM : The film’s budget was about twenty million Euros, with a 12-week shooting schedule. But really, nothing would have been possible without the four months of preparation I mentioned at the start of our discussion. This was a decisive choice to enable everything to fit into our budget.

(Interview conducted by François Reumont, and translated from French by A. Baron-Raiffe)

Sous la Seine
Directed by : Xavier Gens
Produced by : Vincent Roget
Executive Producer : Daniel Delume
Cinematographer : Nicolas Massart, AFC
Set Design : Hubert Pouille
Costume Design : Camille Janbon
Sound : Jacques Sans
Editing : Riwanon Le Beller
Music : Anthony d’Amario, Alex Cortes, Edouard Rigaudière