To create this different type of explosion, we first sculpted the full cave section -floor, wall, and ceiling- in 'green death' urethane carving foam, and we hard-coated it. A large center panel, comprising the full, blown-out center area was then carefully removed and molded with silicone rubber and plaster. Multiple center panels were cast using a weakened A-B foam, that was softened with talc to the point of crumbliness. 

Each fragile piece was placed back in the cavewall center for each take, with cracks along join edges sealed up with oil clay and touch-up painted. In order to spread out the shatter pattern out as much as possible, we used a big metal cone fitting on the air cannon. We rigged the cone with a tethered plywood disc wood plug sealed into the cone with clay and filled over with pea gravel. In order to increase on-set safety, we even individually coated the pea gravel with A-B foam.

As with all of our shatter and explosion effects, film was shot at high speed to capture each split-second take. The camera filmed from behind a sheet of clear lexan, and a large screen of thick plastic sheeting was draped around the lexan to protect the camera crew who also wore face shields. Everyone else watched either from a view point upstairs or outside on a monitor. Even with the foam coating on the pea gravel, we found a few holes in the tough, thick clear plastic sheet barrier. Getting the right explosion involved adjusting and finding the right air pressure and timing release interval for this setup.