The miniature effects were by Grant McCune Design. Clark Schaffer was the miniature set designer with Monty Shook the chief model maker. David Stump was the miniatures director of photography. Jeffrey A. Okun was the production’s visual effects supervisor.
The entire underwater environment was built at 1/16 scale, 40 ft by 20 ft (12m x 6m). It included the habitat base, the sea floor, coral reef and a forced perspective alien spacecraft with a 16 ft (4.9m) tall fin. Plywood was used for the under structure, and coral was represented by spraying polystyrene foam shapes with acetone giving a pitted texture to the surface.
Also included in the sixth scale build was the descent submarine and an escape submarine. The escape submarine featured a pyrex glass sphere for its cockpit bubble as well as a complete interior with sixth scale costumed occupants.
The arrival of the descent submarine at the habitat was achieved by mounting the 1/6th submarine model on an arm attached to a dolly at its stern. It was then moved into shot with the large habitat section casting its miniature lights onto the submarine’s surface.
Virtually all the models were filmed dry in a smoke filled studio for the underwater look, with CG drifting particulate added later in compositing.
The scenes of the escape craft surfacing were the only wet shots filmed at a tank at Universal Studios called Falls lake. Filmed with two high speed cameras by Pete Romano, the model was originally mounted on a surfacing rig but it was found that the escape craft had to be pushed by hand out of the water to achieve the speed required. The background ships were painted plywood shapes mounted on stands at the back of the tank.
While I have no criticism of the miniatures themselves I feel the production design by Norman Reynolds generally leaves a little to be desired in this film. As far as the miniatures go the habitat suffers the most as being very unconvincing as an underwater structure. I feel the production as a whole ( as flawed as it ultimately was ) would have benefited from some decent sci-fi concept design to start with.
Source: Cinefex 74 July 1998, DVD special features.