A rigid transparent ice sheet was required by special effects to create the 'trapped under ice' shoot. Worked quite effectively. The tank that it was fitted to belonged to a Vancouver based company that had developed a deep dive suit and used the tank for testing. When we were to measure up the tank, there was a female Canadian astronaut training in it. Can't remember her name though, very nice Quebecois lady. Rocky shore line with cave - another 'trapped by water' thing, this set piece ended up in another experimental water tank. This one at the university of British Columbia research facility in Vancouver.