They can't get out. They're buried under tons of earth.
3 seconds sound clip from the It Came from Outer Space movie soundboard.
You can hear this line at 01:01:04.211 in the Blu-ray version of the movie.
Quote context
[...]
- She's safe, as long as we don't interfere.
- Maybe that's the way you want it, but not me.
- I want Ellen and the rest of them out now.
- You think I would've walked off and left her if I didn't have to?
- They're leaving tonight. They crashed here by mistake, and now they're fixing their ship to get out!
- They can't get out. They're buried under tons of earth.
- I tell you, she's safe as long as you don't interfere!
- Then why don't they come out in the open?
- Because they don't trust us. Because what we don't understand, we want to destroy.
- I kill only what tries to kill me.
- This.
- Exactly as you'd destroy anything you didn't understand.
[...]