Recreating the ten-minute Trinity nuclear test sequence for Oppenheimer in-camera, to preserve the quality of the film stock, was a big challenge for special effects supervisor Scott Fisher and visual effects supervisor Andrew Jackson and involved a lot of experimentation. The sequence took several weeks to film and the process was quite fragmented. How it was actually pulled off remains a closely guarded secret.
“We created science experiments. We built aquariums with power in it. We dropped silver particles in it. We had molded metallic balloons which were lit up from the inside. We had things slamming and smashing into one another such as ping-pong balls, or just had objects spinning.
We had long shutter speeds, short shutter speeds, wide negative color, negative overexposure, underexposure. It was like a giant playground for all of us,”
Hoyte van Hoytema, cinematographer, Oppenheimer
Source: Variety