Unknown World is a 1951 movie about a trip to the center of the earth. That's not giving anything away, it's all in the poster. It's supposed to be shocking and daring but don't believe it. It didn't live up to it's hype even when it was new. It looks pretty amateurish. There are some effects but they aren't that great. There's not much reason to watch it at all.
The Wikipedia says that some of the story elements are similar to the story of The Core. That's another fairly poor movie that is watchable. So is Unknown World but it's still no excuse to watch it.
Here's a better movie to wash the taste of UW out of your mouth. Fiend Without A Face is a 1958 British science fiction movie that was good enough to make to Criterion's list of films. I've always liked it and have seen it several times. That's the nice thing about some of these old films, they have rewatchability. The movie is set in Canada in the province of my birth but it was filmed in Surrey England. I always wondered why they picked Manitoba.
It's got Marshall Thompson, some guy in a hat, a woman in a towel and those invisible floating brains. What, you can see it? Lucky you, it won't be able to sneak up on you and strangle you with it's highly flexible, and stranglely, spinal column. Here's the description of the movie from the IMDb: A scientist, experimenting with telekinetic powers enhanced by a nearby nuclear power plant succeeds in creating a new form of life. This new creature grows in intelligence until it finally escapes his laboratory. Once outside the lab, and closer to its nuclear power source it multiplies. The creature is also invisible, so no one knows what it looks like.
The scientist's creations eventually become visible so Marshall and the Air Force can fight them off. Luckily you can kill them with an axe. It's all works out pretty well in the end. The movie is directed by Arthur Crabtree and written by Herbert J Leder. Neither men are giants in their field but they turn in a nice workable monster movie that's well worth seeing. I would heartily recommend it.