Clic film with Doug McClure in it?

I remember watching this film years ago, i have recently found out that one of the actors was Doug McClure. I know it involved them going into what they called " the bell" which was a big br dome thing that they lowered into the sea and then they somehow ended up being taken down to another world then stuff happens then they get back in the bell and make it out. Any help or clues that i could chase on google to find it would be cool. Thanks