Remember the T-1000 from Terminator 2; Throughout the film there were only 3,5 minutes CGI, and the scenes where the bullets hit the liquid metal of the terminator were made without the use of computer graphics. In the 90's the graphics were very expensive and their processing took months. So, the special effects team created the 'craters' with chrome-plated foam rubber. The craters were pressed into a fiberglass container, which closed with a kind of lid which opened by remote control. There are no photos of the device itself, but you can see how it worked in one of the rehearsals.