I was going to give this movie a 5 and say, "This is a solid 5!" Then I decided that if it's such a solid 5, why don't I give it a 6?
Why?, you may ask.
I watched it on FX cable last night, having missed the first 10 minutes or so. They were already trapped in the cavern. In a few minutes I was hooked. I happened across it again a few hours later just after the credits and caught the first few minutes until I was up with where I came in. Then I almost sat to watch it again. That alone says something.
Let's get some things out of the way about this film. This is in the public domain. It was shot in 1.85:1, but was broadcast in 4:3. It was broadcast on an HD channel, but still looked as if I was watching it through a silk screen. It may have been a 16mm transfer or a multigenerational copy.
Do not judge a book by its cover.
Many of the comments here about weakness in the script, character motivations and low budget production values are reasonable, but with a caveat. Many of those bad character decisions or odd motivations take place after being trapped in the cavern for weeks and months. When that is taken into account, some of the irrationality might be excused, or at least rationalized.
The cast is actually quite good, though half are likely unknown to most viewers.
The early exterior shots are not encouraging. The opening few minutes which set up the story look low budget. One German soldier stopped his motorcycle to give a report to an officer. After the report, he couldn't restart it, but they kept the "take".
Once the main characters are trapped in the cavern, however, the low budget matters less than the story.
Not everyone survives, and heroes and not-so-much heroes may not be who you expect. But that's a large part of what makes the film engaging.
It won't be the best ~80 minutes of your life, but it won't be a waste.
PS: One of these actors went on to a long-term gig as host of "Hollywood Squares". See if you can spot him. Another went on to be an iconic character on the TV series "Dallas". And this will be one of Brian Aherne's last roles, and not bad though certainly not his best.