The Night the World Exploded (1957)
A 1950's B sci-fi flick where Science Girl has top billing over Science Guy? Maybe that's because she's played by the very pretty, petite, and ultra-feminine Kathryn Grant, who was the Princess in The 7th Voyage of Sindbad. She's in love with Science Guy, who is too wrapped up in his work to notice. On the rebound, she's going to marry some guy named Brad, who never actually shows up. In the movie's least surprising line, she explains "I'm a scientist, but I'm also a woman."
Science Guy's new gizmo predicts a huge earthquake that shows up pretty quick, thanks to lots of stock footage. Things are even worse than they seem. Quakes are shaking things up all over the place, even tilting the crust a few degrees. Our two science types go down into Carlsbad Caverns to see what's going on deep underground. At first, Science Girl freaks out, while descending a rope ladder, but she gets hold of herself and makes it all the way down without help. You go, girl! It turns out that a new element is down there. The stuff expands, heats up, and finally blows up real good when exposed to the nitrogen in the air. Water renders it harmless. (Apparently it's been wet underground all this time, but something - the first quake? -- exposed it.) All the nations of the world work together to flood areas where this stuff is showing up. (Good luck with that, climate change.) The dramatic ending comes when a huge volcano pops up out of nowhere in the desert in the American southwest. Science Guy uses some of the stuff to blow up a dam to flood the place, but not without Science Girl insisting that she go along. Science Guy finally figures out that she's been carrying a torch for him all this time, and humanity is saved. (Those two events aren't necessarily related.)
It's a cheap little film, sort of like a low budget anticipation of the later, much more expensive, and pretty darn good film Crack in the World. Despite being talky and sedate, with much of the story carried by stock footage, a narrator, and those always helpful newspaper headlines, it's not bad for those of us who feast on 1950's SF cheapies.