Days before its U.S. opening, the president of Screen Gems greenlit not only a sequel to the movie Underworld but also a prequel after seeing the number of tickets sold in Canada alone.
Scott Speedman suffered a concussion when a set prop - that was supposed to be a piece of the wall Viktor had just thrown him through - hit him in the head.
There were no special effects used to deepen Kevin Grevioux's (Raze's) voice. His natural speaking voice really is that deep.
(at around 37 mins) The 'attack dogs' outside of the vampire mansion were actually very docile and playful canines, which were the only dogs available at the time. For the scene where they chase Scott Speedman, director Len Wiseman had to film short clips of the dogs running and later put in sounds of vicious barking. If you look closely though you can see their tails happily wagging back and forth.
Not only in this movie did Kate Beckinsale meet her future husband, director Len Wiseman, but at the time she was in a long term relationship with actor Michael Sheen, who played Lucian. Michael and Kate share a daughter together who appears in the film during a flashback of when Selene was younger. Michael and Len have since become very close friends and have collaborated with each other on Underworld: Rise of the Lycans (2009). Michael and Kate still remain very close and both are raising their daughter equally. Kate has stated that Michael is her best friend and that they still remain very close.