El secreto del abismo

Real oxygenated fluorocarbon fluid was used in the rat fluid breathing scene. Dr. Johannes Kylstra and Dr. Peter Bennett of Duke University pioneered this technique and consulted on the film, giving detailed instructions on how to prepare the fluid. The only reason for cutting to the actors' faces was to avoid showing the rats defecating from momentary panic as they began breathing the fluid.

When the divers are swimming, you can see bubbles with every breath, meaning that they are on an open-system (i.e. scuba). However at that depth (2000ft), a standard scuba tank would be exhausted in under two minutes, or much less with any kind of exertion.

In the cast list, Super Seal Rover is credited as Big Geek and Mini Rover Mk II is credited as Little Geek. These are the actual models used for the unmanned submarines.

The end credits were famously shortened to run under 5 minutes in 1989 in order to hit a target runtime and maximize daily showings; doing so also made the crawl almost illegibly tiny and fast. The credits on the extended edition were almost 10 minutes, with a bigger and slower crawl, and extended/alternate music. Several home video releases of the theatrical edition on laserdisc and DVD actually use the newer credits, so they are not entirely faithful! One VHS tape (Fox Video Selections 1561), at least, uses the original short credits; though the tape is formatted for 4:3, the credits merely gain picture information above and below the intended window (which, for some reason, is very high up in the frame after the first few names). However the new HD master, which has popped up on a few HDTV broadcasts (like Cinemax) go back to the original shortened credits, plus is in the original theatrical aspect ratio, making it the most faithful version available.

Edited into No Feat But What We Make: 'Terminator 2' and the Rise of Digital Effects (2003)