The final episode of Under Salt Marsh airs tonight, and it brings the terrifying storm that has been building all season straight into the homes of viewers. The Sky Atlantic crime drama, set in the fictional Welsh town of Morfa Halen, has kept audiences guessing who killed eight-year-old Cefin. But the real star of the finale might just be the water.
Production designer Stevie Herbert recently sat down with Radio Times to explain exactly how the team managed to flood an entire village set for Episode 6. The answer involves a massive steel tank, thousands of litres of water, and a lot of very careful planning .
A 50-Metre Tank Built Just for One Scene
Flooding a real town in North Wales was never an option. It is too dangerous, too expensive, and completely impossible to control. So the production team did the next best thing. They built their own version of Morfa Halen from scratch.
“We built the whole center of the village,” Herbert explained. The team constructed an exterior backlot of the town’s main shopping area just outside Dragon Studios in Bridgend. This was not a small set. It measured 50 x 50 metres and was surrounded by steel walls standing 1.2 metres tall .
Creating a tank that could hold water without leaking everywhere took serious effort. The crew used timber, glue, and expanding foam to seal every gap. They even wrapped tarmac around the edges to make sure the water stayed exactly where it was supposed to be. The goal was to create a controlled environment where the cast and stunt teams could work safely while nature (or at least a very good imitation of it) did its worst .
A Week of Filming with Rising Water Levels
The flood sequence in Episode 6 did not happen in one day. It took a full week to capture every moment. Water arrived on set in massive bowsers—large tanker trucks—and was pumped slowly into the village.
“We built the whole center of the village” specifically to manage this process. The filming schedule was built around the water levels. Shooting started when the water was just 20 centimetres deep. As the week went on, the team let more water in, eventually reaching almost one metre .
This slow flood allowed the actors to perform scenes at different stages of the disaster. Viewers will see the village go from wet streets to full-on flooding as the storm peaks. The production team also installed rain rigs above the entire set to make sure everything stayed soaked.
To make the water look real, special effects crews added pumps to create a strong current. Flood water is never still, and the team needed that dangerous movement to sell the threat. Herbert and her props team spent long hours inside the tank themselves. “I was dressing the set at different stages, so I had full waders on with my props team,” she said. They worked alongside professional dive teams to make sure everything remained safe while the cameras rolled .
Making Water Safe for Cast and Crew
Working in deep water for a week creates unique problems. Some objects float. Others sink and become tripping hazards. The team had to think carefully about every single item placed in the village.
Plastic chairs, bowls, and pots were allowed to float upside down because that looks natural in a flood. But anything dangerous or likely to cause electrical problems was removed entirely. Shop units were stripped down so there were no moving parts inside that could fail when wet .
There was another major concern: the water itself. The sets sat in that water for seven days. If the water contained toxic chemicals, it would be dangerous for the actors and crew working in it. It could also harm the environment when it was time to drain the tank.
Herbert confirmed that everything used to build the village was safe. “The steels were clean, we used natural materials to build walls, which were all timber and ply-based, and the paint treatments had to be water-based,” she said. After filming finished, the water was released slowly back into the normal system with no risk of pollution .
Why the Flood Had to Feel Real
The storm in Under Salt Marsh is not just background noise. It is a central part of the story. The rising water cuts Morfa Halen off from the outside world. It threatens to destroy evidence. It puts every character in danger. And in Episode 6, it leads to the discovery of another body .
Herbert knew the flood had to look completely believable for the drama to work. “It’s not easy to imitate nature,” she said. “The storm and the flooding being believable was the biggest challenge. You’ve got to feel the fear, but you’ve also got to feel the potential loss of all these people” .
The production team built the whole center of the village to make sure audiences felt that fear. Every wave, every current, every floating object had to be exactly right. The water had to look like it could actually sweep someone away.
Filming in Real Wales Locations
While the flood was built in a tank, the series did use real Welsh locations for other scenes. Barmouth, a seaside town on the coast, served as one of the primary filming spots. The team also shot in and around Cardiff to capture the moody, unpredictable nature of the Welsh coastline .
These real locations helped establish the world of Morfa Halen. They showed viewers why this town is so vulnerable to the sea. But for the climax, the team needed total control. Only a purpose-built tank could deliver the scale and danger required for the finale.
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The Finale Arrives Tonight
Under Salt Marsh Episode 6 premiered February 27, on Sky Atlantic at 9pm. It is available to stream on NOW in the UK .
The episode brings the investigation to its conclusion. With the town cut off by floodwaters and a killer still on the loose, time is running out for everyone. Residents are evacuating. Roads are disappearing underwater. And somewhere in the chaos, the truth about Cefin’s death—and possibly what happened to Nessa years ago—is waiting to be found.
Herbert and her team have built a disaster that feels terrifyingly real. Now viewers finally get to see if anyone makes it out alive.
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