Not the Abyss

[April]

All I remember about this movie is that people are at the bottom of the ocean and they find a door and go in it. Everything is white, they get "decontaminated" and there's like a whole clan of people that live there. Any ideas?

[Carl]

Is it "Captain Nemo and the Underwater City" (1969)?

http://www.imdb.com/title/tt0065522/

[April]

No, it's not that old.

[Ethan Bourne]

What you describe is very similar to Robin Cook's book Abduction (2000).  Some of his books have been made into movies, but I don't believe this has.  Here's a description:

A team of marine experts are abducted into a world under the oceans where highly evolved humans have been living for millions of years. They must first be decontaminated and then integrated/introduced to their new lives and to these "peaceful" people who have found the secret to eternal life and heaven on earth. Are they captive or can they return to surface earth and their families?  

[April]

That sounds really close, but it was never made into a movie :( I just spent half an hour googling everything I could think of, but no luck. This is driving me crazy!

[Ethan Bourne]

It would help to know the decade of the movie.  In any case, two other possibilities are: Goliath Awaits (1981) where divers go through an underwater portal and discover an entire colony of 300 people living under the ocean in a sort of utopian community.  Another is Dark Descent (2002), where two investigators visit an under sea mining colony at the bottom of the ocean (in the Marianas Trench in the South Pacific) ... the movie is similar to "Outland" (with Sean Connery), except that, instead of outer space, the mining colony is underneath the ocean floor.

[Knaldskalle]

Lords of the Deep?

[Xro]

Do not think it sounds right with a Clan of people living there but maybe "Sphere"

http://www.imdb.com/title/tt0120184/