Pat Boone actually gets top billing in this movie but most people probably remember James Mason as the star, with Pat playing the support. He was really in there as box office insurance, singing a few songs - and it did so well that you wonder why Pat didn't make more science fiction/adventure movies, where he didn't have to stress about being immoral.
Because this is probably the best film old Pat ever made - something you could probably also say for director Henry Levin, who has a lot of bland films on his resume. It's a really top notch, classy adventure movie, with plenty of impressive production values, good acting and scripy, and sense of wonder. Everyone brought their A game - Arlene Dahl is great as well (this is probably the best film she ever made too.
A lot of credit surely must go to Charles Brackett who produced and co-wrote. It's a tasteful, intelligent adaptation of Verne - there are some large creatures but they are saved until the end, the filmmakers don't go overboard. There's no lost native civilisation which means no romance underground for Boone and/or Peter Ronson, but does make things more believable.
The world underground is genuinely creepy and feels authentic - caverns, large mushrooms, smouldering volcanoes, dusty floors, the 300 year old dead body of an explorer, inland seas, whirlpools. It feels isolated, dangerous, scary. (I've always remembered the sounds of this film - the gushing of the water, the silence).
There is excitement in that Mason is given a rival scientist... then a more ruthless one (Thayer David) who tries to take over the mission. Dahl adds a welcome dash of romance although the Mason/Dahl "I don't want a woman coming along"/"I insist" banter gets tired quickly (I did like how they have Dahl's character status by having her the one who can translate Icelandic for Ronson).
Ronson and Boone both kind of have the same role - to run around shirtless (Boone's physique is really exploited in this movie - he even has a gratuitous water fall washing scene). Ronson doesn't have much of a character, though he is center stage for the most emotionally affecting scene in the film - when the goose is killed. (The fact the goose is actually killed by David as opposed to it just being threatened is what makes this movie top class.) Boone has a great moment where he's kind of hitting on Dahl and get reminds him to back off, in a nice way - this is good writing.
It's just all extremely well done and 20th Century Fox never managed to match it again though they tried several times eg The Lost World, Five Weeks in a Balloon.