The other three were Lords of the Deep, The Rift (or Endless Descent as it was known in Europe), and The Evil Below.

The Rift didn't release until 1990, but it was greenlit in '89 after the release of Leviathan. Dino De Laurentiis, who produced Leviathan, decided for some reason he wanted to finance a really low-budget remake after seeing the original. A really low-budget remake. Leviathan had a budget of $30 million -- The Rift got $1.3 million to work with.

It's absolutely terrible, despite having R. Lee (Full Metal Jacket) Ermey and Ray (Twin Peaks) Wise in the main cast. It makes Leviathan look like Aliens by comparison. :)