Most of Raiders Of The Lost Ark's action is stunt work and physical FX. However, when it came to the climatic opening of the Ark, it was a little more dependent on visual effects. George Lucas' pioneering FX house Industrial Light & Magic, under the supervision of Richard Edlund, were called upon to create the startling power of the Ark. "It was Steven at his virtuoso best," said Edlund "but it made our job that much more difficult. Most of our work consisted of optical effects for the last reel of the film and included one of the most difficult subjects to portray -- ghosts, eerie, scary, believable ghosts."

The film's visual effects art director Joe Johnston, who has since gone on to be a successful film director himself, remembers that this FX-laden sequence was more than a little vague going in. "At first, Lucas and Spielberg did not really know what the end sequence was going to consist of. The script said something like: The Nazis open the Ark and there is a giant cataclysm! It wasn't quite that nonspecific, but it was almost that bad. There were ten of us involved in these early conferences -- three of us were artists. We were told to try to imagine what would happen when the Nazis open the Ark, which us supposed to be the repository of an awesome power. We could do anything we wanted to; though we knew that spirits or manifestation of spirits was involved, George and Steven weren't sure how they should appear on the screen. Each of the artists did a preliminary set of storyboards on their own. For this film, having three artists at work on the same sequence was a unique and interesting way to work, since we all came up with totally different things. My early thoughts had no ghosts at all -- it was all firestorm. Another artist had all ghosts and no flame; the third had all these weird light effects."

Originally, the ghosts were to be created with cel animation, but the early tests didn't satisfy the filmmakers. So instead, Edlund and company used the cloud tank for Close Encounters Of The Third Kind to aid them. It was refurbished and tests were shot of the ghosts in water. This made the spirits appear to be somewhat formless and floating through the atmosphere. Four shots of ghosts in the finished film were done through cel animation, most of the spiritual FX was done through miniatures in the cloud tank, puppets and actors.

The tank went on to become a fixture of FX movies, but at the time was still very fresh. "You create an inversion layer in the tank using different temperatures and densities of solutions," explained Edlund. "For example a layer of salt water on the bottom of the tank with a layer of fresh water above it. You can float various pigments and dyes in the plane where the two layers meet thereby generating different types of cloud  effects. We use what we call an 'atomic arm' (a remote-controlled hand, such as is used for moving isotopes in nuclear laboratories) to squirt pigment into the tank at the appropriate level. It is designed so you can control the insertion of the pigment from back near the camera, so you can see pretty much what the camera sees as you make a shot. The cloud tank has now become part of our repertoire."

As the power of the Ark is unleashed, the Nazi officers are hit by lightening bolts, that spreads from person to person. Although the bolts are added following filming with visual effects, the glowing lights on each body was done live. "We built a special harness for the Nazi's that housed an enormous flashbulb," said Edlund. "The trigger was concealed in the hands of the individual stuntmen. On cue the stuntmen fired the flashbulb, which lit up the entire inside of their shirts, so that it looked as if something was actually burning inside. Of course the stuntmen were protected with asbestos pads. But the effect...was effective!"

The villainous trio of Toht, Belloq and Dietrich suffer considerably nastier, and gorier, fates. It's undoubtable a pretty shocking piece of FX, and it's surprisingly that Spielberg was able to get it into a PG rated movie. The most famous FX piece in this sequence was the melting of Toht. An uncanny prop head of actor  Ronald Lacey was created by Tom Smith and his crew, and a heat lamp was used to melt the coating, reducing it to a sculptured skull. For Dietrich, a hollow head was crafted, and the air was sucked out of it, making it cave in on itself. Belloq's exploding head was made of plaster and filled with blood. However,  Belloq's fate was such a brutal image originally, Spielberg had to add a pillar of flame in front in order for Raiders to actually avoid the R rating. It's now seen rather vaguely, but it still works as a pretty horrific moment.

With the Nazi's all dead, the power of the Ark sweeps across the island, creating a fire storm, which Edlund discussed. "Our first challenge involved finding a means of controlling flames and fire into storyboarded shapes. Thaine Morris, our resident pyrotechnican, was pretty successful. Most of the material was shot with Bruce Hill's highspeed Photosonics camera. We shot at 360 frames per second in four-perf anamorphic format. The VistaVision equipment is not capable of such high speeds. I built a little optical printer to convert the four-perf anamorphic footage to VistaVision interpositive, so we could composite the material on our VistaVision Quad printer.

"Paul Hustin built a miniature of the mountain of the mountain top set about four feet wide and five or six feet deep for the shots in which the firestorm sweeps over the scene. We turned the miniature upside down in order to get the flames to cling to the set."

The climax of Raiders Of The Lost Ark is a true classic. It's exciting, unpredictable and pretty damn frightening. It's also a sequence that showed how a vague script description could be brilliantly thought out and executed by the effects and makeup department. For their remarkable efforts, Edlund, Johnston, Kit West (mechanical effects supervisor) and Bruce Nicholson (optical photography supervisor) received the Oscar for Visual Effects.